Palæospondylus.—According to Dr. Traquair: "The Palæospondylus gunni is a very small organism, usually under one inch in length, though exceptionally large specimens occasionally measure one inch and a half.... It has a head and vertebral column, but no trace of jaws or limbs; and, strange to say, all the specimens are seen only from the ventral aspect, as is shown by the relation of the neural arches to the vertebral centra.

"The head is in most cases much eroded.... It is divided by a notch ... into two parts.... The anterior part shows a groove the edges of which are elevated, while the surface on each side shows two depressions, like fenestræ, though perhaps they are not completely perforated, and also a groove partially divided off, posteriorly and externally, a small lobe. In front there is a ring-like opening ... surrounded by small pointed cirri, four ventrally, at least five dorsally, and two long lateral ones which seem to arise inside the margin of the ring instead of from its rim like the others. The posterior part of the cranium is flattened, but the median groove is still observable. Connected with the posterior or occipital aspect of the skull are two small narrow plates which lie closely alongside the first half-dozen vertebræ.

"The bodies of the vertebræ are hollow or ring-like, and those immediately in front are separated from each other by perceptible intervals; their surfaces are marked with a few little longitudinal grooves, of which one is median. They are provided with neural arches, which are at first short and quadrate, but towards the caudal extremity lengthen out into slender neural spines, which form the dorsal expansion of a caudal fin, while shorter hæmal ones are also developed on the ventral aspect."

Dr. Traquair concludes that "there seems to be no escape from the conclusion that the little creature must be classed as a Marsipobranch." "If Palæospondylus is not a Marsipobranch, it is quite impossible to refer it to any other existing group of vertebrates."

Gill on Palæospondylus.—In 1896 Dr. Gill proposed to regard Palæospondylus provisionally as the type of a distinct order of Cyclostomes to be called Cycliæ (κύκλος, circle), from the median ring on the head, whether nostril or mouth. Dr. Gill observes:

"Assuming the correctness of Dr. Traquair's description and figures, we certainly have a remarkable combination of characters. On the one hand, if the 'median opening or rim' is indeed nasal, the animal certainly cannot be referred to the class of Selachians or of Teleostomes. On the other hand, the cranium and the segmental vertebral column indicate a more advanced stage of development of the vertebrate line than that from the living Marsipobranchs must have originated. We may, therefore, with propriety isolate it as the representative not only of a peculiar family (Palæospondylidæ), but of an order or even subclass (Cycliæ) of vertebrates which may provisionally (and only provisionally) be retained in the class of Marsipobranchs.

"The group may be defined as Monorrhines with a continuous (?) cranium, a median nasal (?) ring, and a segmented vertebral column.

"The differences between the Hyperoartia and Hyperotreta are very great, and Prof. Lankester did not go much too far when he elevated those groups to class rank. Among the numerous distinctive characters are the great differences in the auditory organs. Perhaps the organs of Palæospondylus might be worked out in some specimen and throw light on the subject of affinities. At present even the region of the auditory organs is not exactly known and we are now at a loss to orient the several parts of the cranium. In fact, the question of the relations of Palæospondylus is a very open one."

Views as to the Relationships of Palæospondylus.—Dr. Dean thus summarizes in a convenient and interesting fashion the views of different students of fossil fishes in regard to Palæospondylus:

Huxley.—A "baby Coccosteus."

Traquair, 1890.—"Certainly not a Placoderm, its resemblance to a supposed 'baby Coccosteus' being entirely deceptive. The appearance of the head does remind us in a strange way of the primitive skull of Myxine, a resemblance which is rendered still more suggestive by the apparent complete absence of the lower jaw, or of limbs or limb-girdles."

Traquair, 1893.—"It seems, indeed, impossible to refer the organism to any existing vertebrate class, unless it be the Marsipobranchs or Cyclostomata." Does not believe it a larval form, because the possible adult is unknown, and because of the highly differentiated vertebræ. Granting his interpretation of the parts of the fossil, "there seems no escape from the conclusion that the little creature must be classed as a Marsipobranch."

Traquair, 1897.—"The question of the affinities of Palæospondylus is left precisely where it was after I had written my last paper on the subject."

Smith Woodward, 1892.—"It seems to possess an unpaired nose, lip cartilages in place of functional jaws, and no paired limbs; thus agreeing precisely with the lampreys and hagfishes, of which the fossil representatives have long been sought. It is extremely probable, therefore, that Palæospondylus belongs to this interesting category."

Dawson, 1893.—Palæospondylus suggests "the smaller snake-like Batrachians of the Carboniferous and Permian; and I should not be surprised if it should come to be regarded as either a forerunner of the Batrachians or as a primitive tadpole."

Gill, 1896.—"The group to which Palæospondylus belongs may be defined as Monorrhines with a continuous (?) cranium, a median nasal (?) ring, and a segmented vertebral column." "The cranium and segmented vertebral column indicate a more advanced stage of development of the vertebrate line than that from which the living Marsipobranchs must have originated. We may, therefore, with propriety isolate it as the representative not only of a peculiar family (Palæospondylidæ), but of an order or even subclass (Cycliæ) of vertebrates which may provisionally (and only provisionally) be retained in the class of Marsipobranchs."

Dean, 1896.—"Place it with the Ostracoderms among the curiously specialized offshoots of the early Chordates, but this position would be at the best unsatisfactory."

Dean, 1898.—"Palæospondylus should not be given a place—even a provisional one—among the Marsipobranchs." To be accepted "as the representative of the new subclass (or class) Cycliæ constituted for it by Professor Gill."

Parker & Haswell, 1897.—"There is some reason to regard that Palæospondylus is referable to the Cyclostomes." "A distinctly higher type than recent forms."

Gegenbaur, 1898.—"Discovery of Palæospondylus one of the highest importance. If this organism stands in no way near the Cyclostomes, the tentacles lose their higher importance, since they also occur in other groups." "Through Palæospondylus came also the attempt (Pollard) to deduce the presence of the tentacular condition in the higher forms." (Mem.—In this Gegenbaur has not consulted the literature accurately. At the time of founding his "Cirrhostomal Theory" Pollard was unaware of the discovery of Palæospondylus). "Ich muss sagen, das die positive Behauptung der einen wie der anderen Deutung mir sehr unsicher scheint, da auch an den übrigen Resten des Kopfskelets keine bestimmten Uebereinstimmungen mit anderen Organismen erweisbar sind. Es ist daher auch nicht zu vermuthen, dass sogar an Beziehung zu Froschlarven gedacht ward. Unter diesen Umständen möchte ich jene im Verhältniss zum Kopfe wie zum gesammten Körper bedeutende, von Cirren umstellte Eingangsöffnung als nicht einer Nase, sondern einem Munde oder beiden zugleich angehörig betrachten. Zu einem dem Cyclostomenriechorgan vergleichbaren Verhalten fehlen alle Bedingungen."

Relationships of Palæospondylus.—The arguments for and against the supposition that Palæospondylus is a Cyclostome may be here summed up after Professor Dean.

The vertebral column agrees with that of the lamprey in having the notochord in part persistent. On the other hand, the vertebræ have continuous centra, showing definite processes. Those of the different regions are differentiated. These conditions are quite unlike those seen in the lamprey.

The cranium is massive, over twice as large proportionally as that of the lamprey. In the latter type the cranium forms but a small portion of the bulk of the head; in Palæospondylus, on the other hand, the cranium bears every sign of having filled the contour of the head. Moreover, if the region adjacent to the structure is admitted to be that of the eye, and few, I believe, will doubt it, then the brain-cavity must, by many analogies, have been much larger than that of a Marsipobranch. Also the auditory capsules must have been of extraordinary size. In short, there is very little about the cranium to suggest the structures of Cyclostomes.

The "oral cirri" suggest somewhat the barbels of the nose and mouth of a hagfish. They, however, resemble even as much in arrangement and greater number the buccal cirri of Amphioxus. On the other hand, similar mouth-surrounding tentacles are evolved independently in many groups of fishes, siluroids, sharks, forms like Pogonias, Hemitripterus. A possibility further exists that the "cirri" may turn out to be remnants of cranial or facial structures of an entirely different nature. In fact the very uncertain preservation of these parts renders their evidence of little definite value. In but one specimen, as far as I am aware, is there any evidence of the presence of ventral cirri.

The jaw parts in Palæospondylus are unknown. It is possible that the ventral rim of the "nasal ring" may prove to be the remains of the Meckelian cartilage (the cartilaginous core of the lower jaw).

It is possible that certain very faint ray-like markings noted by Professor Dean may be the basalia of paired fins. In such case Palæospondylus can have no affinity with the lampreys. Dr. Dean asserts that the presence of these, in view of the wide dissimilarity in other and important structures, is sufficient to remove Palæospondylus from its provisional position among the Cyclostomes. The postoccipital plates may represent a pectoral arch. It is, however, much more likely, as Dr. Traquair has insisted, that the supposed rays are due to the reflection of light from striations on the stone, and that the creature had no pectoral limbs.

The caudal fin, with its dichotomous rays, is essentially like the tail of a lamprey. This condition is, however, found in other groups of fishes, as among sharks and lung-fishes. It is, moreover, doubtful whether the rays are really dichotomous.

It is possible that Palæospondylus may be, as Huxley suggests, a larval Arthrodire. It is not probable that this is the case, but, on the other hand, Palæospondylus seems to be an immature form. According to Dr. Dean, it is more likely to prove a larval Coccosteus, or the young of some other Arthrodire, than a lamprey. Against this view must be urged the fact that the tail of Palæospondylus is not heterocercal, a fact verified by Dr. Traquair on all of his many specimens. It is more like the tail of a lamprey than that of Coccosteus. It is, however, certain that it cannot be placed in the same class with the living Cyclostomes, and that it is far more highly specialized than any of them. In a still later paper (1904) Dr. Dean shows that the fossil might as easily be considered a Chimæra as a lamprey, and repeats his conviction that it is a larval form of which the adult is still unrecognized.

We cannot go much farther than Dr. Dean's statement in 1896, that it belongs "among the curiously specialized offshoots of the early Chordates."

CHAPTER XXXIV
THE CROSSOPTERYGII

Class Teleostomi.—We may unite the remaining groups of fishes into a single class, for which the name Teleostomi (τέλεος, true; στόμα, mouth), proposed by Bonaparte in 1838, may be retained. The fishes of this class are characterized by the presence of a suspensorium to the mandible, by the existence of membrane-bones (opercles, suborbitals, etc.) on the head, by a single gill-opening leading to gill-arches bearing filamentous gills, and by the absence of claspers on the ventral fins. The skeleton is at least partly ossified in all the Teleostomi. More important as a primary character, distinguishing these fishes from the sharks, is the presence typically and primitively of the air-bladder. This is at first a lung, arising as a diverticulum from the ventral side of the œsophagus, but in later forms it becomes dorsal and is, by degrees, degraded into a swim-bladder, and in very many forms it is altogether lost with age.

This group comprises the vast majority of recent fishes, as well as a large percentage of those known only as fossils. In these the condition of the lung can be only guessed.

The Teleostomi are doubtless derived from sharks, their relationship being possibly nearest to the Ichthyotomi or to the primitive Chimæras. The Dipnoans among Teleostomi retain the shark-like condition of the upper jaw, made of palatal elements, which may be, as in the Chimæra, fused with the cranium. In the lower forms also the primitive diphycercal or protocercal form of tail is retained, as also the archipterygium or jointed axis of the paired fins, fringed with rays on one or both sides.

We may divide the Teleostomes, or true fishes, into three subclasses: the Crossopterygii, or fringe-fins; the Dipneusti, or lung-fishes; Actinopteri, or ray-fins, including the Ganoidei and the Teleostei, or bony fishes. Of these many recent writers are disposed to consider the Crossopterygii as most primitive, and to derive from it by separate lines each of the remaining subclasses, as well as the higher vertebrates. The Ganoidei and Teleostei (constituting the Actinopteri) are very closely related, the ancient group passing by almost imperceptible degrees into the modern group of bony fishes.

Subclass Crossopterygii.—The earliest Teleostomes known belong to the subclass or group called after Huxley, Crossopterygii (κρόσσος, fringe; πτερύξ, fin). A prominent character of the group lies in the retention of the jointed pectoral fin or archipterygium, its axis fringed by a series of soft rays. This character it shares with the Ichthyotomi among sharks, and with the Dipneusti. From the latter it differs in the hyostylic cranium, the lower jaw being suspended from the hyomandibular, and by the presence of distinct premaxillary and maxillary elements in the upper jaw. In these characters it agrees with the ordinary fishes. In the living Crossopterygians the air-bladder is lung-like, attached by a duct to the ventral side of the œsophagus. The lung-sac, though specialized in structure, is simple, not cellular as in the Dipnoans. The skeleton is more or less perfectly ossified. Outside the cartilaginous skull is a bony coat of mail. The skin is covered with firm scales or bony plates, the tail is diphycercal, straight, and ending in a point, the shoulder-girdle attached to the cranium is cartilaginous but overlaid with bony plates, and the branchiostegals are represented by a pair of gular plates.

In the single family represented among living fishes the heart has a muscular arterial bulb with many series of valves on its inner edge, and the large air-bladder is divided into two lobes, having the functions of a lung, though not cellular as in the lung-fishes.

The fossil types are very closely allied to the lung-fishes, and the two groups have no doubt a common origin in Silurian times. It is now usually considered that the Crossopterygian is more primitive than the lung-fish, though at the same time more nearly related to the Ganoids, and through them to the ordinary fishes.

Origin of Amphibians.—From the primitive Crossopterygii the step to the ancestral Amphibia, which are likewise mailed and semi-aquatic, seems a very short one. It is true that most writers until recently have regarded certain Dipneustans as the Dipteridæ as representing the parents of the Amphibians. But the weight of recent authority, Gill, Pollard, Boulenger, Dollo, and others, seems to place the point of separation of the higher vertebrates with the Crossopterygians, and to regard the lobate pectoral member of Polypterus as a possible source of the five-fingered arm of the frog. This view is still, however, extremely hypothetical and there is still much to be said in favor of the theory of the origin of Amphibia from Dipnoans and in favor of the view that the Dipnoans are also ancestors of the Crossopterygians.

In the true Amphibians the lungs are better developed than in the Crossopterygian or Dipnoan, although the lungs are finally lost in certain salamanders which breathe through epithelial cells. The gills lose, among the Amphibia, their primitive importance, although in Proteus anguineus of Austria and Necturus maculosus, the American "mud-puppy" or water-dog, these persist through life. The archipterygium, or primitive fin, gives place to the chiropterygium, or fingered arm. In this the basal segment of the archipterygium gives place to the humerus, the diverging segments seen in the most specialized type of archipterygium (Polypterus) become perhaps radius and ulna, the intermediate quadrate mass of cartilage possibly becoming carpal bones, and from these spring the joints called metacarpals and phalanges. In the Amphibians and all higher forms the shoulder-girdle retains its primitive insertion at a distance from the head, and the posterior limbs remain abdominal.

The Amphibians are therefore primarily fishes with fingers and toes instead of the fringe-fins of their ancestors. Their relations are really with the fishes, as indicated by Huxley, who unites the amphibians and fishes in a primary group, Ichthyopsida, while reptiles and birds form the contrasting group of Sauropsida.

The reptiles differ from the Amphibians through acceleration of development, passing through the gill-bearing stages within the egg. The birds bear feathers instead of scales, and the mammals nourish their young by means of glandular secretions. Through a reptile-amphibian ancestry the birds and mammals may trace back their descent from palæozoic Crossopterygians. In the very young embryo of all higher vertebrates traces of double-breathing persist in all species, in the form of rudimentary gill-slits.

The Fins of Crossopterygians.—Dollo and Boulenger regard the heterocercal tail as a primitive form, the diphycercal form being a result of degradation, connected with its less extensive use as an organ of propulsion. Most writers who adopt the theory of Gegenbaur that the archipterygium is the primitive form of the pectoral fin are likely, however, to consider the diphycercal tail found associated with it in the Ichthyotomi, Dipneusti, Crossopterygii as the more primitive form of the tail. From this form the heterocercal tail of the higher sharks and Ganoids may be derived, this giving way in the process of development to the imperfectly homocercal tail of the salmon, the homocercal tail of the perch, and the isocercal tail of the codfish and its allies, the gephyrocercal and the leptocercal tail, tapering or whip-like, representing various stages of degeneration. Boulenger draws a distinction between the protocercal tail, the one primitively straight, and the diphycercal tail modified, like the homocercal tail, from an heterocercal ancestry.

Orders of Crossopterygians.—Cope and Woodward divide the Crossopterygia into four orders or suborders, Haplistia, Rhipidistia, Actinistia, and Cladistia. To the latter belong the existing species, or the family of Polypteridæ, alone. Boulenger unites the three extinct orders into one, which he calls Osteolepida. In all three of these the pectorals are narrow with a single basal bone, and the nostrils, as in the Dipneustans, are below the snout. The differences are apparently such as to justify Cope's division into three orders.

Haplistia.—In the Haplistia the notochord is persistent, and the basal bones of dorsal and anal fins are in regular series, much fewer in number than the fin-rays. The single family Tarrassiidæ is represented by Tarrasius problematicus, found by Traquair in Scotland. This is regarded as the lowest of the Crossopterygians, a small fish of the Lower Carboniferous, the head mailed, the body with small bony scales.

Rhipidistia.—In the Rhipidistia the basal bones of the median fins ("axonosts and baseosts") are found in a single piece, not separate as in the Haplistia. Four families are recognized, Holoptychiidæ, Megalichthyidæ, Osteolepidæ, and Onychodontidæ, the first of these being considered as the nearest approach of the Crossopterygians to the Dipnoans.

The Holoptychiidæ have the pectoral fins acute, the scales cycloid, enameled, and the teeth very complex. Holoptychius nobilissimus is a very large fish from the Devonian. Glyptolepis leptopterus from the Lower Devonian is also a notable species. Dendrodus from the Devonian is known from detached teeth.

In the Ordovician rocks of Cañon City, Colorado, Dr. Walcott finds numerous bony scales with folded surfaces and stellate ornamentation, and which he refers with some doubt to a Crossopterygian fish of the family Holoptychiidæ. This fish he names Eriptychius americanus. If this identification proves correct, it will carry back the appearance of Crossopterygian fishes, the earliest of the Teleostome forms, to the beginning of the Silurian, these Cañon City shales being the oldest rocks in which remains of fishes are known to occur. In the same rocks are found plates of Ostracophores and other fragments still more doubtful. It is certain that our records in palæontology fall far short of disclosing the earliest sharks, as well as the earliest remains of Ostracophores, Arthrodires, or even Ganoids.

Megalichthyidæ.—The Megalichthyidæ (wrongly called "Rhizodontidæ") have the pectoral fins obtuse, the teeth relatively simple, and the scales cycloid, enameled. There are numerous species in the Carboniferous rocks, largely known from fragments or from teeth. Megalichthys, Strepsodus, Rhizodopsis, Gyroptychius, Tristichopterus, Eusthenopteron, Cricodus, and Sauripterus are the genera; Rhizodopsis sauroides from the coal-measures of England being the best-known species.

The Osteolepidæ differ from the Megalichthyidæ mainly in the presence of enameled rhomboid scales, as in Polypterus and Lepisosteus. In Glyptopomus these scales are sculptured, in the others smooth. In Osteolepis, Thursius, Diplopterus, and Glyptopomus a pineal foramen is present on the top of the head. This is wanting in Parabatrachus (Megalichthys of authors). In Osteolepis, Thursius, and Parabatrachus the tail is heterocercal, while in Diplopterus and Glyptopomus it is diphycercal. Osteolepis macrolepidotus and numerous other species occur in the Lower Devonian. Diplopterus agassizii is common in the same horizon. Megalichthys hibberti is found in the coal-measures, and Glyptopomus minimus in the Upper Devonian. Palæosteus is another genus recently described.

The Onychodontidæ are known from a few fragments of Onychodus sigmoides from the Lower Devonian of Ohio and Onychodus anglicus from England.

Order Actinistia.—In the Actinistia there is a single fin-ray to each basal bone, the axonosts of each ray fused in a single piece. The notochord is persistent, causing the back-bone in fossils to appear hollow, the cartilaginous material leaving no trace in the rocks. The genera and species are numerous, ranging from the Subcarboniferous to the Upper Cretaceous, many of them belonging to Cœlacanthus, the chief genus of the single family Cœlacanthidæ. In Cœlacanthus the fin-rays are without denticles. Cœlacanthus granulatus is found in the European Permian. Cœlacanthus elegans of the coal-measures is found in America also. In Undina the anterior fin-rays are marked with tubercles. Undina penicillata and Undina gulo from the Triassic are well-preserved species. In Macropoma (lewesiensis) the fin-rays are robust, long, and little articulated. Other genera are Heptanema, Coccoderma, Libys, Diplurus, and Graphiurus. Diplurus longicaudatus was found by Newberry in the Triassic of New Jersey and Connecticut.

Order Cladistia.—In the Cladistia the axis of the pectoral limb is fan-shaped, made of two diversified bones joined by cartilage. The notochord is restricted and replaced by ossified vertebræ. The axonosts of the dorsal and anal are in regular series, each bearing a fin-ray. The order contains the single family Polypteridæ. In this group the pectoral fin is formed differently from that of the other Crossopterygians, being broad, its base of two diverging bones with cartilage between. This structure, more specialized than in any other of the Crossopterygians or Dipneusti, has been regarded by Gill and others, as above stated, as the origin of the fingered hand (chiropterygium) of the frogs and higher vertebrates. The base of the diverging bones has been identified as the antecedent of the humerus, the bones themselves as radius and ulna, while the intervening non-ossified cartilage breaks up into carpal bones, from which metacarpals and digits ultimately diverge. This hypothesis is open to considerable doubt. The nostrils, as in true fishes, are superior. The body in these fishes is covered with rhombic enameled scales, as in the garpike; the head is similarly mailed, but, in distinction from the garpike, the anterior rays of the dorsal are developed as isolated spines.

The young have a bushy external gill with a broad scaly base. The air-bladder is double, not cellular, with a large air-duct joining the ventral surface of the œsophagus. The intestine has a spiral valve.

The cranium, according to Boulenger ("Poissons du Bassin du Congo," p. 11), is remarkable for its generalized form, this character forming a trait of union between the Ganoids and the primitive Amphibia or Stegocephali. Without considering Polypterus, it is not possible to interpret the homologies of the cranium of the amphibians and the sharks.

The jaws are similar to those of the vertebrates higher than fishes. Tooth-bearing premaxillaries and dentaries are solidly joined at the front of the cranium, and united by a suture to the toothed maxillaries which form most of the edge of the mouth. Each half of the lower jaw consists of four elements, covering Meckel's cartilage, which is ossified at the symphysis. These are the articular, angular, dentary, and splenial (coronoid). Most of these bones are armed with teeth. The palato-suspensory consists of hyomandibular, quadrate, ectopterygoid, entopterygoid, metapterygoid, and palatine elements, the pterygoid elements bearing teeth. In Erpetoichthys only the opercle is distinct among the gill-covers. In Polypterus there is a subopercle also; the suborbital chain is represented by two small bones.

The gill-arches are four, but without lower pharyngeals. The teeth are conic and pointed, and in structure, according to Agassiz, they differ largely from those of bony fishes, approaching the teeth of reptiles.

The external gill of the young, first discovered by Steindachner in 1869, consists of a fleshy axis bordered above and below by secondary branches, themselves fringed. In form and structure this resembles the external gills of amphibians. It is inserted, not on the gill-arches, but on the hyoid arch. Its origin is from the external skin. It can therefore not be compared morphologically with the gills of other fishes, nor with the pseudobranchiæ, but rather with the external gills of larval sharks. The vertebræ are very numerous and biconcave as in ordinary fishes. Each of the peculiar dorsal spines is primitively a single spine, not a finlet of several pieces, as some have suggested. The enameled, rhomboid scales are in movable oblique whorls, each scale interlocked with its neighbors.

The shoulder-girdle, suspended from the cranium by post-temporal and supraclavicle, is covered by bony plates. To the small hypercoracoid and hypocoracoid the pectoral fin is attached. Its basal bones may be compared to those of the sharks, mesopterygium, propterygium, and metapterygium, which may with less certainty be again called humerus, radius, and ulna. These are covered by flesh and by small imbricated scales. The air-bladder resembles the lungs of terrestrial vertebrates. It consists of two cylindrical sacs, that on the right the longer, then uniting in front to form a short tube, which enters the œsophagus from below with a slit-like glottis. Unlike the lung of the Dipneusti, this air-bladder is not cellular, and it receives only arterial blood. Its function is to assist the respiration by gills without replacing it.

The Polypteridæ.—All the Polypteridæ are natives of Africa. Two genera are known, no species having been found fossil. Of Polypterus, Boulenger, the latest authority, recognizes nine species: six in the Congo, Polypterus congicus, P. delhezi, P. ornatipinnis, P. weeksi, P. palmas, and P. retropinnis; one, P. lapradei, in the Niger; and two in the Nile, Polypterus bichir and P. endlicheri. Of these the only one known until very recently was Polypterus bichir of the Nile.

These fishes in many respects resemble the garpike in habits. They live close on the mud in the bottom of sluggish waters, moving the pectorals fan-fashion. If the water is foul, they rise to the surface to gulp air, a part of which escapes through the gill-openings, after which they descend like a flash. In the breeding season these fishes are very active, depositing their eggs in districts flooded in the spring. The eggs are very numerous, grass-green, and of the size of eggs of millet. The flesh is excellent as food.

The genus Erpetoichthys contains a single species, Erpetoichthys calabaricus,[163] found also in the Senegal and Congo. This species is very slender, almost eel-like, extremely agile, and, as usual in wriggling or undulating fishes, it has lost its ventral fin. It lives in shallow waters among interlaced roots of palms. When disturbed it swims like a snake.

CHAPTER XXXV
SUBCLASS DIPNEUSTI,[164] OR LUNGFISHES

The Lungfishes.—The group of Dipneusti, or lung-fishes, is characterized by the presence of paired fins consisting of a jointed axis with or without rays. The skull is autostylic, the upper jaw being made as in the Chimæra of palatal elements joined to the quadrate and fused with the cranium, without premaxillary or maxillary. The dentary bones are little developed. The air-bladder is cellular, used as a lung in all the living species, its duct attached to the ventral side of the œsophagus. The heart has many valves in the muscular arterial bulb. The intestine has a spiral valve. The teeth are usually of large plates of dentine covered with enamel, and are present on the pterygo-palatine and splenial bones. The nostrils are concealed, when the mouth is closed, under a fold of the upper lip. The scales are cycloid, mostly not enameled.

The lung-fishes, or Dipneusti (δίς, two; πνεῖν, to breathe), arise, with the Crossopterygians, from the vast darkness of Palæozoic time, their origin with that or through that of the latter to be traced to the Ichthyotomi or other primitive sharks. These two groups are separated from all the more primitive fish-like vertebrates by the presence of lungs. In its origin the lung or air-bladder arises as a diverticulum from the alimentary canal, used by the earliest fishes as a breathing-sac, the respiratory functions lost in the progress of further divergence. Nothing of the nature of lung or air-bladder is found in lancelet, lamprey, or shark. In none of the remaining groups of fishes is it wholly wanting at all stages of development, although often lost in the adult. Among fishes it is most completely functional in the Dipneusti, and it passes through all stages of degeneration and atrophy in the more specialized bony fishes.

In the Dipneusti, or Dipnoans, as in the Crossopterygians and the higher vertebrates, the trachea, or air-duct, arises, as above stated, from the ventral side of the œsophagus. In the more specialized fishes, yet to be considered, it is transferred to the dorsal side, thus avoiding a turn in passing around the œsophagus itself. From the sharks these forms are further distinguished by the presence of membrane-bones about the head. From the Actinopteri (Ganoids and Teleosts) Dipnoans and Crossopterygians are again distinguished by the presence of the fringe-fin, or archipterygium, as the form of the paired limbs. From the Crossopterygians the Dipnoans are most readily distinguished by the absence of maxillary and premaxillary, the characteristic structures of the jaw of the true fish. The upper jaw in the Dipnoan is formed of palatal elements attached directly to the skull, and the lower jaw contains no true dentary bones. The skull in the Dipnoans, as in the Chimæra, is autostylic, the mandible articulating directly with the palatal apparatus, the front of which forms the upper jaw and of which the pterygoid, hyomandibular and quadrate elements form an immovable part. The shoulder-girdle, as in the shark, is a single cartilage, but it supports a pair of superficial membrane-bones.

In all the Dipnoans the trunk is covered with imbricated cycloid scales and no bony plates, although sometimes the scales are firm and enameled. The head has a roof of well-developed bony plates made of ossified skin and not corresponding with the membrane-bones of higher fishes. The fish-like membrane-bones, opercles, branchiostegals, etc., are not yet differentiated. The teeth have the form of grinding-plates on the pterygoid areas of the palate, being distinctly shark-like in structure. The paired fins are developed as archipterygia, often without rays, and the pelvic arch consists of a single cartilage, the two sides symmetrical and connected in front. There is but one external gill-opening leading to the gill-arches, which, as in ordinary fishes, are fringe-like, attached at one end. In the young, as with the embryo shark, there is a bushy external gill, which looks not unlike the archipterygium pectoral fin itself, although its rays are of different texture. In early forms, as in the Ganoids, the scales were bony and enameled, but in some recent forms deep sunken in the skin. The claspers have disappeared, the nostrils, as in the frog, open into the pharynx, the heart is three-chambered, the arterial bulb with many valves, and the cellular structure of the skin and of other tissues is essentially as in the Amphibian.

The developed lung, fitted for breathing air, which seems the most important of all these characters, can, of course, be traced only in the recent forms, although its existence in all others can be safely predicated. Besides the development of the lung we may notice the gradual forward movement of the shoulder-girdle, which in most of the Teleostomous fishes is attached to the head. In bony fishes generally there is no distinct neck, as the post-temporal, the highest bone of the shoulder-girdle, is articulated directly with the skull. In some specialized forms (Balistes, Tetraodon) it is even immovably fused with it. In a few groups (Apodes, Opisthomi, Heteromi, etc.) this connection ancestrally possessed is lost through atrophy and the slipping backward of the shoulder-girdle leaves again a distinct neck. In the Amphibians and all higher vertebrates the shoulder-girdle is distinct from the skull, and the possession of a flexible neck is an important feature of their structure. In all these higher forms the posterior limbs remain abdominal, as in the sharks and the primitive and soft-rayed fishes generally. In these the pelvis or pelvic elements are attached toward the middle of the body, giving a distinct back as well as neck. In the spiny-rayed fishes the "back" as well as the neck disappears, the pelvic elements being attached to the shoulder-girdle, and in a few extreme forms (as Ophidion) the pelvis is fastened at the chin.

Classification of Dipnoans.—By Woodward the Dipneusti are divided into two classes, the Sirenoidei and the Arthrodira. We follow Dean in regarding the latter as representative of a distinct class, leaving the Sirenoidei, with the Ctenodipterini, to constitute the subclass of Dipneusti. The Sirenoidei are divided by Gill into two orders, the Monopneumona, with one lung, and the Diplopneumona, with the lung divided. To the latter order the Lepidosirenidæ belong. To the former the Ceratodontidæ, and presumably the extinct families also belong, although nothing is known of their lung structures. Zittel and Hay adopt the names of Ctenodipterini and Sirenoidei for these orders, the former being further characterized by the very fine fin-rays, more numerous than their supports.

Order Ctenodipterini.—In this order the cranial roof-bones are small and numerous, and the rays of the median fins are very slender, much more numerous than their supports, which are inserted directly on the vertebral arches.

In the Uronemidæ the upper dentition comprises a cluster of small, blunt, conical denticles on the palatine bones; the lower dentition consists of similar denticles on the splenial bone. The vertical fins are continuous and the tail diphycercal. There is a jugular plate, as in Amia. The few species are found in the Carboniferous, Uronemus lobatus being the best-known species.

In Dipteridæ there is a pair of dental plates on the palatines, and an opposing pair on the splenials below. Jugular plates are present, and the tail is usually distinctly heterocercal.

In Phaneropleuron there is a distinct anal fin shorter than the very long dorsal; Phaneropleuron andersoni is known from Scotland, and Scaumenacia curta is found at Scaumenac Bay in the Upper Devonian of Canada.

In Dipterus there are no marginal teeth, and the tail is heterocercal, not diphycercal, as in the other Dipnoans generally. Numerous species of Dipterus occur in Devonian rocks. In these the jugular plate is present, as in Uronemus. Dipterus valenciennesi is the best-known European species. Dipterus nelsoni and numerous other species are found in the Chemung and other groups of Devonian rocks in America.