Fam. 5. Muraenidae.—Maxillaries absent, replaced by the palato-pterygoid, the mouth bordered by the latter and the ethmo-vomer; palato-pterygoid bone separated from hyomandibular arch; branchial openings into the pharynx narrow slits; no tongue.

The body is naked, pectoral fins are usually absent, and the gill-cleft is a small round opening. The opercular bones are much reduced in size, and the pectoral arch may be totally absent.

Voracious marine Fishes, inhabiting tropical and subtropical waters, being especially abundant about coral reefs. Some 120 species are known, many reaching a very large size, and being also remarkable for their variegated coloration. The genera are mostly founded on the dentition, which shows much diversity; the following are the principal:—Myroconger, Enchelycore, Muraena, Thyrsoidea, Lycodontis, Pythonichthys, Echidna, Channomuraena. The Muraena of the ancient Romans, Muraena helena, of the Mediterranean, Eastern Atlantic, and neighbouring parts of the Indian Ocean, occurring exceptionally as far north as the English coast, grows to 4 feet, and its flesh was more esteemed than that of any other Fish, large numbers being reared in specially constructed reservoirs near the sea, and fed on the corpses of slaves. Channomuraena vittata, from the coast of Cuba, is known to attain a length of 8 feet, and Thyrsoidea macrura, from the Indian Ocean and the Malay Archipelago, to upwards of 10 feet.

Sub-Order 5. Haplomi.

Air-bladder, if present, communicating with the digestive tract by a duct. Opercle well developed. Pectoral arch suspended from the skull; no mesocoracoid arch. Fins usually without, rarely with a few spines; ventrals abdominal, if present. Anterior vertebrae distinct, without Weberian ossicles.

The absence of the mesocoracoid arch distinguishes the Haplomi from the Malacopterygii, with which they are united by various authors. They lead to the Percesoces through the Cyprinodontids, and to the Lower Acanthopterygians, such as the Berycidae, through the Scopelids, Stephanoberycids, and Percopsids, as is evidenced by the structure of the mouth and the forward position, in some of the genera, of the ventral fins, which, however, are never attached to the pectoral girdle. Most of the forms which are here included inhabit either fresh water or the deep sea.

Synopsis of the Families.

Fam. 1. Galaxiidae.—Margin of the upper jaw formed by the praemaxillaries and the maxillaries, the latter behind the former, and toothless. Parietals in contact with each other, and separating the frontals from the supraoccipital; opercular bones all well developed. Basis cranii simple. Ribs inserted on strong, autogenous parapophyses; epipleurals and epineurals. Post-temporal simple, attached to the epiotic; post-clavicle present. Body naked. Vertical fins far back; no adipose dorsal fin. Pectoral fins inserted very low down. Ventrals, if present, with seven rays. Air-bladder present. Ova falling into the cavity of the abdomen before exclusion.

The genus Galaxias has an interesting distribution, the species of which it is made up occurring in the fresh waters of the southern hemisphere, viz. 8 in New Zealand and neighbouring islands, 7 in New South Wales, 3 or 4 in South Australia, 1 in West Australia, 2 in Tasmania, 7 in South America, from Chili southwards, and 1 at the Cape of Good Hope. One species (G. attenuatus) is even believed to be identical in New Zealand, Tasmania, South Australia, the Falkland Islands, and South America. This conclusion is probably correct from the fact, which may account for the distribution of the whole genus, that it is not confined to fresh waters, but occurs also in the sea. Specimens were observed by Mr. Rupert Vallentin in the Falkland Islands, where the Fish is known to the inhabitants as "Smelts," in shoals in the shallow water along the shore; and, according to Mr. F. E. Clarke, the same species, in New Zealand, periodically descends to the sea, where it spawns, from January to March, and returns from March to May. A marine species has recently been discovered at the Chatham Islands. In New Zealand, the Galaxias were called "Trout" by the settlers before the introduction of Salmonids, whilst the fry of G. attenuatus are eaten as "Whitebait." The largest species reach the length of a foot. Neochanna, from New Zealand, differs from Galaxias in the absence of ventral fins; it has been found in burrows, which it excavates at a distance from water.

Fam. 2. Haplochitonidae.—Small fresh-water Trout-like Fishes, agreeing in most respects with the Galaxiidae, to which they are unquestionably closely related, differing only in the greater extent of the praemaxillaries, which exclude the maxillary from the oral border, in the double basis cranii (the prootics uniting under the brain, leaving a canal between them and the parasphenoid), in the shorter parapophyses, which, like the neural arches of the praecaudal vertebrae, are autogenous, and in the presence of a small adipose dorsal fin, opposed to the anal.

Two genera: Haplochiton, naked, with a single species from Chili, the southern extremity of South America, and the Falkland Islands, and Prototroctes, covered with small scales, of which one species inhabits Queensland, another South Australia, and a third New Zealand. In Haplochiton, the urogenital orifice of both sexes is produced into a cylindrical tube, which lies concealed in a groove in front of the anal fin.

Fam. 3. Enchodontidae.—Margin of the upper jaw formed by the praemaxillaries and the maxillaries, the latter sometimes toothed like the former. Frontals in contact with the supraoccipital; basis cranii simple. Ribs sessile; praecaudal vertebrae without transverse processes. Rayed dorsal fin never much extended; sometimes an adipose fin behind it. Scales delicate or absent, but occasional longitudinal series of scutes occur, the dorsal series, when present, being unpaired.

Cretaceous Fishes allied to, and apparently more generalised than, the Esocidae and Scopelidae. Numerous remains from Europe and North America, referred to 7 genera: Enchodus, Eurypholis, Palaeolycus, Halec, Cimolichthys, Prionolepis.

Fam. 4. Esocidae.—Margin of the upper jaw formed by the praemaxillaries and maxillaries, the latter behind the former, and toothless. Supraoccipital in contact with the frontals, separating the small parietals; opercular bones all well developed; basis cranii simple. No parapophyses, except to the hindermost praecaudal vertebrae; epipleurals and epineurals. Post-temporal forked, the upper branch attached to the epiotic, the lower to the exoccipital; post-clavicle present. Vertical fins far back; no adipose dorsal fin. Pectoral fins inserted very low down; ventrals with 6 to 11 rays. Air-bladder present.

As in the Haplochitonidae, the neural and haemal arches are bones distinct from the centra, and although parapophyses are not developed, the ribs are not inserted on the centra, but on distinct bases wedged into the latter.^[691] Teeth are present on the vomer, palatine, and pharyngeal bones.

A small family of carnivorous freshwater Fishes, including the Pike (Esox), of predaceous habits, unsurpassed in greediness and voracity, and the small and insignificant-looking Umbra, distinguished by the more anterior position of the dorsal fin, the larger scales, and the moderately large gape, with feeble villiform teeth. The range of the Esocidae is restricted to the cold and temperate parts of the northern hemisphere. Besides the well-known Esox lucius of Europe, Northern Asia, and the northern parts of North America, growing to a length of 4 feet, and the Maskinongy (E. nobilior) of north-eastern North America, reaching twice that length, the first genus comprises three smaller species from the Eastern United States. Remains of Esox have been found in various freshwater deposits in Europe as far back as the Oligocene. E. lepidotus, of which very perfect specimens have been found in the Upper Miocene beds of Oeningen in Baden, differs from the living species in its much larger scales and in the greater approximation of the ventral and anal fins, two characters in which it approaches Umbra. Only two species of the latter are known: U. crameri ("Hundsfisch"), from the stagnant waters of Austria-Hungary, and U. limi ("Mud-Fish"), living in swamps and ditches in Canada and the north-eastern United States, often remaining imbedded in the mud of prairie sloughs and bog-holes.

Fam. 5. Dalliidae.—The genus Dallia, with a single species inhabiting the streams and ponds of Alaska and Siberia, is related to Umbra, but differs in the very thin and papery skeleton, with the post-temporal imperfectly ossified and the pectoral fin without pterygials or actinosts. The dorsal fin is far back and opposite to the anal, as in the Pike. The ventral fins are composed of three rays only, and the pectorals, which have a somewhat fleshy base, have as many as 36. The scales are extremely small, and partly imbedded in the skin. The Black-Fish, D. pectoralis, abounds in Sphagnum ponds, feeding on plants and worms, and forming the chief food of the natives of some parts of Northern Alaska, where, with the exception of the Salmonids, it is the only freshwater Fish. Turner, its discoverer, says its vitality is extraordinary: Black-Fishes will remain frozen in baskets for weeks, and when thawed are as lively as ever, one having been swallowed in a congealed condition by a dog, thawed out by the heat of the stomach, and vomited up alive.

Fam. 6. Scopelidae.—Praemaxillaries much elongate, and completely excluding the maxillaries from the oral border. Supraoccipital in contact with the frontals, sometimes partly covered by the parietals; opercular bones all well developed. Basis cranii simple. Ribs sessile; no parapophyses on the praecaudal vertebrae; epipleurals and epineurals. Post-temporal forked, the upper branch in contact with the epiotic or the supraoccipital, the lower with the opisthotic; post-clavicle present. An adipose dorsal fin often occurs; luminous spots often present on head and body. Ventral fins with 7 to 10 rays. Air-bladder sometimes absent.

A large family (over 100 known living species), mostly of pelagic and deep-sea Fishes. A great number of fossil types have been described.

Recent genera:

A. Without photophores: Saurus, Saurida, Bathysaurus, Harpodon, Scopelarchus, Aulopus, Odontostomus, Omosudis, Sudis, Paralepis, Bathypterois, Benthosaurus, Chlorophthalmus, Ipnops. B. With photophores: Scopelus, Dasyscopelus, Neoscopelus, Scopelengys, Nannobrachium, Scopelosaurus.

Fossil genera:

A. Cretaceous: Sardinioides, Acrognathus, Leptosomus, Sardinius, Dactylopogon, Nematonotus, Microcoelia, Opisthopteryx, Apateodus, Rhinellus. B. Eocene, Oligocene, and Miocene: Omiodon, Scopeloides, Parascopelus, Anapterus.

The members of this Family vary much in form, and among them are to be found some of the most curious adaptations to bathybial existence. One of the best known is Harpodon nehereus, which, when newly taken, is brilliantly phosphorescent all over the body; in a salted and dry condition it is the "Bombay-duck," a delicacy eaten with curries, and exported in large quantities from the west coast of India. It is not known to occur at any great depth, and is not even restricted to the sea, being very abundant in the rivers and estuaries of Bengal and Burma; whilst an allied species, H. squamosus, is found in the Indian Ocean at depths of 120 to 300 fathoms. In Bathypterois, the eyes are very small; some of the rays of the paired fins being excessively prolonged, acting as tactile organs, and compensating the reduction in the eyes. Sir John Murray has observed about B. longipes: "When taken from the trawl [from 2650 fathoms] they were always dead, and the long pectoral rays were erected like an arch over the head, requiring considerable pressure to make them lie along the side of the body; when erected they resembled Pennatulids like Umbellula." In Ipnops, which resembles in general form the large-eyed Chlorophthalmus gracilis, the upper surface of the broad spatulate snout is occupied by a luminous organ longitudinally divided into two symmetrical halves, and the eyes are absent, unless, as first supposed, this extraordinary organ be a modification of them; but Professor Moseley's examination seems to have proved beyond doubt that it is a special form of phosphorescent organ, the object of which would be to attract other creatures to the wide gape of a Fish which, living in the abysses of the sea and deprived of organs of sight and touch, would have great difficulty in procuring its food. Odontostomus, with a very large eye which can be turned upwards and sidewards, and enormous compressed curved teeth, barbed at the tip and depressible backwards, is one of the few Scopelids in which scales are completely absent.

The numerous species (about 50) of Scopelus and their allies are moderate-sized or small pelagic and deep-sea forms found in nearly all the seas, some coming to the surface at night, whilst others are confined to great depths; they are remarkable for the series of phosphorescent spots (photophores) on the body, and in some species also on the head, where they may form large patches on the snout. The arrangement of these photophores is a very definite one, and it has been used for the division of these Fishes into genera or sub-genera.^[692] The ventral fins have a more forward position than in most other members of the Family.

Fam. 7. Alepidosauridae.—Characters as in the preceding, but supratemporal simple, attached to the opisthotic, and dorsal fin very long, formed of slender, non-articulated, simple or bifid rays, extending along nearly the whole length of the back, followed by a small adipose fin. The air-bladder is absent and the body scaleless. The skeleton is feebly ossified; the dentition is very powerful, some of the teeth on the palate and mandible being very strongly enlarged. 4 or 5 species are known, from considerable depths in the Atlantic and Pacific Oceans, referable to one genus, Alepidosaurus or Plagyodus. A. ferox, from the Atlantic, reaches a length of 4 feet.

Fam. 8. Cetomimidae.—The affinities of the recently discovered genera Rondeletia and Cetomimus, deep-sea Fishes from the North Atlantic, at depths of 1000 to 1600 fathoms, are still uncertain, as the skeleton could not be examined; they are probably most nearly related to the Scopelidae. The head is enormous, with very wide gape, that of Cetomimus being suggestive of that of a Right Whale; the teeth are small and coarsely granular; the gill-openings are very wide; the body is more or less compressed and scaleless; the dorsal and anal fins are opposed to each other; no adipose dorsal fin. In Rondeletia, the eyes are moderately large, and ventral fins, with 5 rays, are present; in Cetomimus, the eyes are very small, and ventral fins are absent.

Fam. 9. Chirothricidae.—Praemaxillaries delicate and styliform, completely excluding the maxillaries from the upper border of the mouth; jaws with feeble dentition or toothless; opercular apparatus complete. Praecaudal vertebrae with robust parapophyses, to which ribs are attached. Ventral fins far forwards.

These Fishes, of which three fossil genera are known from the Cretaceous of Germany and Syria, appear to be related to the Scopelidae, from which the strong parapophyses distinguish them. Chirothrix is remarkable for its excessively enlarged ventral fins with about 17 rays; these fins were taken for the pectorals by the early describers. In Telepholis and Exocoetoides, the ventral fins are smaller than the pectorals, and formed of 7 or 8 rays only; the dorsal region, in the former, is protected by a covering of small, thin, rounded or polygonal dermal scutes, each bearing a median tubercle.

Fam. 10. Kneriidae.—Margin of the upper jaw formed by the praemaxillaries; mouth toothless, not protractile. Parietals separated by the supraoccipital. Pharyngeal bones toothless. Praecaudal vertebrae with parapophyses. Body covered with small scales. Ventrals with 9 rays. No adipose dorsal fin. Air-bladder present.

The genus Kneria comprises two species from the fresh waters of tropical Africa, one from Angola, the other from East Africa.^[693] Small Loach-like Fishes, two to four inches long, with the upper jaw projecting beyond the mouth, which is inferior and transverse; no barbels; gill-membranes entirely grown to the isthmus, the gill-opening being a rather narrow vertical slit; dorsal and anal fins short, the former opposite, or nearly opposite, to the ventrals; the snout of the male(?) of K. angolensis is described as beset with small spine-like excrescences; the intestinal tract makes several convolutions.

Fam. 11. Cyprinodontidae.—Mouth protractile, the maxillaries excluded from the oral border; teeth in the jaws and on the pharyngeal bones; pterygo-palatine arch weak or rudimentary; opercular bones all well developed. Basis cranii simple. Praecaudal vertebrae with strong parapophyses, bearing the ribs; epipleurals inserted on the ribs. Post-temporal forked. Ventrals, if present, with 5 to 7 rays. No adipose dorsal fin. Air-bladder sometimes absent.

From a physiological point of view, this Family may be divided into carnivorous forms, with short digestive tract, and phytophagous or limnophagous ones, in which the intestine forms numerous coils. To the first division belong the living genera Cyprinodon, Characodon, Tellia, Haplochilus, Fundulus, Rivulus, Cynolebias, Orestias, Empetrichthys, Jenynsia, Pseudoxiphophorus, Belonesox, Gambusia, Anableps, among existing forms, and the fossil genera Prolebias (Oligocene and Miocene) and Pachylebias (Miocene); to the second, the living genera Poecilia, Mollienesia, Platypoecilus, and Girardinus. Procatopus, a near ally of Haplochilus, recently discovered in South Cameroon, is remarkable for having the ventral fins inserted far forward, below the pectoral fins.

These are small or very small Fishes,^[694] only a few reaching a length of about a foot, confined to fresh or brackish waters, recognisable externally by the flat head with protractile mouth, the usually large scales, and the absence of a well-developed lateral line. The teeth vary much in shape: cardiform, villiform, or compressed, and bi- or tri-cuspid; the palate is either toothless, or teeth are present on the vomer. About 200 species are known, mostly from the American continent, only about 30 being known from other parts of the world, viz. Southern Europe, Southern Asia and Japan, and Africa. In many species the sexes are dissimilar, the female being larger and less brilliantly coloured, with smaller fins; the anal fin of the male may be modified into an intromittent organ by means of which internal fertilisation takes place, the ova developing in a sort of uterus, which the young leave in a more or less advanced stage of growth. The most curious of the Cyprinodontids is the genus Anableps, of Central and South America, surface-swimming Fishes, the strongly projecting eyes of which are divided by a horizontal band of the conjunctiva into an upper part adapted for vision in the air, and a lower for vision in the water, and the pupil is divided into two parts by a constriction; the larger species grows to the length of a foot.

Fam. 12. Amblyopsidae.—Mouth scarcely protractile, the maxillaries excluded from the oral border; teeth small, in jaws and palate, and on the pharyngeal bones. Praecaudal vertebrae with very strong parapophyses, bearing the ribs on their upper surface; epipleurals inserted on the ribs. Ventral fins rudimentary or absent. Vent jugular, close to the gill-clefts. Air-bladder present.

Small ovoviviparous Fishes, closely related to, and evidently derived from, the Cyprinodontids, measuring from 1 to 5 inches, inhabiting ditches and small streams, or confined to subterranean waters of limestone caves, in the United States east of the Rocky Mountains. Six species, referable to three genera, are known. In Chologaster, the eyes are well developed and the body is coloured. C. cornutus inhabits the lowland streams and swamps of the South Atlantic States, from Virginia to Florida; C. agassizii is found in the underground streams of Kentucky and Tennessee; and C. papilliferus occurs under stones in the springs of south-western Illinois. Amblyopsis and Typhlichthys, which are evidently derived from the former, or from forms closely related to it, have the eyes rudimentary and more or less concealed under the skin, and the body is colourless. Amblyopsis spelaea is widely distributed in the caves east of the Mississippi, both north and south of the Ohio River; it is common in the River Styx of the Mammoth Cave. Typhlichthys subterraneus is found with the latter species in the caves east of the Mississippi, but is confined to the south side of the Ohio River, whilst T. (Troglichthys) rosae is found in the caves west of the Mississippi River. Of Amblyopsis spelaea, the late Professor Cope has observed: "If these Amblyopses be not alarmed, they come to the surface to feed, and swim in full sight, like white aquatic ghosts. They are then easily taken by the hand or net, if perfect silence is preserved, for they are unconscious of the presence of an enemy except through the medium of hearing; this sense, however, is evidently very acute, for at any noise they turn suddenly downwards, and hide beneath stones, etc., on the bottom." Dr. Garman thinks, on the contrary, that such a sense can hardly be developed in recesses where we are accustomed to think any sounds other than those made by the rippling or dripping water are almost unknown, and that it is through the sense of touch, and not through hearing, that the Fish is disturbed. In fact, the head is provided with a great number of tactile papillae, arranged in transverse ridges, provided with nervous filaments, which evidently compensate the loss of the visual organ.^[695]

Fam. 13. Stephanoberycidae.—This Family has hitherto been placed near the Berycidae, among the Acanthopterygii, but there are no spinous rays in the dorsal and anal fins; and the ventrals, formed of one simple and four or five branched rays, are abdominal. The genus Stephanoberyx, with two species from the Atlantic, at depths of 535 to 2949 fathoms, is characterised by a large, thick, cavernous head, with thin bony spine-bearing ridges, a large mouth bordered by the protractile praemaxillaries, behind which are the large maxillaries, a short dorsal and a short anal, opposed to each other behind the ventrals, and the body covered with feebly imbricated scales, each bearing in the centre one or several erect spines. The largest specimen measures 6 inches. Malacosarcus, a small Fish from the Pacific, at depths of 2350 and 2425 fathoms, is very closely allied to Stephanoberyx, but its scales are very thin and cycloid. The striking resemblance which the head bears to that of the Berycid Melamphaes may be merely a case of convergence, and it must be borne in mind that this appearance is approached by some species of Scopelus, with which both Malacosarcus and Melamphaes were originally confounded. The praecaudal vertebrae are provided with parapophyses. I have ascertained on a specimen of Stephanoberyx monae that the air-bladder is connected with the dorsal side of the stomach by a short and comparatively wide duct.

Fam. 14. Percopsidae.—Margin of the upper jaw formed by the praemaxillaries; mouth small, not protractile, toothed; palate toothless. Supraoccipital in contact with the frontals, separating the small parietals. Basis cranii simple. Most of the praecaudal vertebrae with parapophyses, on the upper surface of which the ribs are inserted; no epipleurals. Post-temporal forked; post-clavicle present; scapular foramen in the scapula, on which three hour-glass-shaped pterygials are inserted, a fourth being inserted on the coracoid. Dorsal fin with two true spines; anal with one or two; ventrals far forward, with 9 rays; pectorals inserted rather high. A small adipose dorsal fin. Body covered with strongly ctenoid scales. Air-bladder present (with open duct).

This is a most interesting group of Fishes, from the resemblance which they bear to the Perches, and they have therefore been raised to the rank of a sub-order, Salmopercae, by Jordan and Evermann, who regard them as "archaic fishes, relics of some earlier fauna, and apparently derived directly from the extinct transitional forms through which the Haplomi and Acanthopteri have descended from allies of the Isospondyli [Malacopterygii]." On the other hand, an analysis of their characters shows them to belong to the Haplomi, of which they may be regarded as highly specialised members, having evolved in the direction of the Acanthopterygii.

Only two genera are known, each with a single species: Percopsis, from the rivers and streams of Canada and the north-eastern United States, and Columbia, more recently discovered in the sandy or weedy lagoons along the Columbia River. These Fishes are of small size, not exceeding 6 inches in length. Their eggs are unusually large.

Sub-Order 6. Heteromi.

Air-bladder without open duct. Opercle well developed; parietal bones separating the frontals from the supraoccipital. Pectoral arch suspended from the supraoccipital or the epiotic, the post-temporal small and simple or replaced by a ligament; no mesocoracoid. Ventral fins abdominal, if present.

The Halosauridae and Notacanthidae are deep-sea Fishes of obscure affinities. In the abdominal position of the many-rayed ventral fins and in the absence of the mesocoracoid arch they agree with the Haplomi; but if, as the investigations of Günther lead us to believe,^[696] there is really no open communication between the air-bladder and the digestive tract, they should be removed from this physostomous sub-order. The two families have many characters in common, such as the attachment and structure of the pectoral arch, which is devoid of a post-clavicle, the position of the pectoral fins high up the sides, the strong parapophyses inserted very low down on the centra of the vertebrae, the extent of the parietal bones, which meet in a sagittal suture and separate the frontals from the supraoccipital. The recent discovery of a third family, the Lipogenyidae, which, in the structure of the dorsal fin, is exactly intermediate between the two others, has lessened the gap between the Lyomeri (Halosauridae) and Heteromi (Notacanthidae) of Gill, which I have proposed to unite in a suborder under the latter name.

These Fishes are no doubt derived from forms in which a separate caudal fin existed; such a type must have been near the Dercetidae, as defined by A. S. Woodward, which may provisionally be placed here.

An imperfectly known Fish from the Chalk of Mount Lebanon, Pronotacanthus sahelalmae, appears to bear some affinity to Notacanthus, and has been placed in the same family; but its characters are not sufficiently defined to refer it without doubt to this division.

There is a fifth family which may enter this sub-order: the Fierasferidae, the structure of which has been exquisitely described and figured by Emery.^[697] Hitherto placed with or near the Ophidiidae, they differ widely from them, as well as from all Acanthopterygians, in the conformation of the skull, the supraoccipital being separated from the frontals by the parietals, which form a long median suture. This is a feature which has only been observed in Fishes with abdominal ventral fins, and although the total absence of those fins in Fierasfer deprive us of an important criterion in deciding on its affinities, I am inclined to regard this family as derived from an "abdominal" type. The conformation of the pectoral arch has much in common with that of the Halosaurs, and, notwithstanding the interpretation that has been given to the bones at the back of the cranium in the latter type, the same may be said, in a general way, of the skull.

As pointed out by Emery, the very anterior position of the vent in the Fierasferidae is directly related to the curious mode of life of these Fishes, and the analogous condition obtained in various families, such as the Gymnotidae, Nemichthyidae, Amblyopsidae, shows it to be a character of relatively small systematic importance.

Synopsis of the Families.

Fam. 1. Dercetidae.—Body much elongate; ordinary scales small or wanting, but two or more continuous series of enlarged scutes along each side; mouth large, praemaxillaries apparently forming the greater part of the upper border of the mouth, which is toothed; opercular apparatus complete. Dorsal fin more or less extended, without spines; anal short, caudal separate; ventrals with not less than 7 or 8 rays.

Dercetis, Leptotrachelus, Leptecodon, Pelargorhynchus, and Stratodus, from the Upper Cretaceous of Europe, Syria, and North America.

Fam. 2. Halosauridae.—Body elongate, covered with cycloid scales, the tail tapering to a point, without caudal fin; head with scales; mouth moderate, bordered by the praemaxillaries and the maxillaries, both toothed; suborbitals large; praeopercle rudimentary. Dorsal fin short, formed of soft rays, above or a little behind the ventrals, which are rather far back, and formed of 9 or 10 rays; anal very long, without spines, extending to the end of the tail. Ovaries transversely laminated, the ova falling into the abdominal cavity. Some 10 living species are known, referred to three genera, inhabiting the Atlantic, Pacific, and Indian Oceans, at depths of 500 to 1400 fathoms.

In Halosaurus the scales of the lateral line, which runs near the lower profile, are scarcely enlarged, and are destitute of luminous organs. Halosaurichthys differs in the union of the ventral fins with each other, as in Notacanthus. In Halosauropsis the scales of the lateral line are strongly enlarged and pouch-like, and bear photophores.

This family is one of great antiquity, being represented in the Upper Cretaceous of Westphalia by Echidnocephalus, which, as shown by A. S. Woodward, appears to have been closely related to Halosaurus.

Fam. 3. Lipogenyidae.—Similar to the preceding in shape and in the position of the dorsal fin, but with a toothless, roundish, inferior, suctorial mouth, and with the short dorsal and the long anal formed partly of spines and partly of soft rays. Head and body covered with minute scales; lateral line nearer the dorsal than the ventral profile. Ventrals with 3 spines and 7 soft rays. A single species, Lipogenys gillii, from the North Atlantic, 865 fathoms.

Fam. 4. Notacanthidae.—Body elongate, covered with very small cycloid scales, the tail tapering to a point, without caudal fin; head scaly; mouth small, inferior, bordered by the praemaxillaries only; jaws toothed; no suborbitals; praeoperculum small; post-temporal replaced by ligament. Dorsal fin formed of a series of short disconnected spines; anal very long, formed partly of spines and partly of soft rays, extending to the end of the tail. Ventrals with 1 to 5 spines and 7 to 10 soft rays.

Two genera: Notacanthus, with the ventrals connate or confluent and with 6 to 12 dorsal spines; and Polyacanthonotus, with the ventrals separated and 27 to 38 dorsal spines. Nine species, from the Mediterranean, the Atlantic, and the Pacific, at depths of 400 to 1875 fathoms.

Fam. 5. Fierasferidae.—Body elongate or extremely attenuate, naked, the tail tapering to a point or truncate, without distinct caudal fin; mouth small, inferior, bordered by the praemaxillaries; jaws toothed; no suborbitals; praeoperculum well developed. Dorsal and anal fins very long, extending to the end of the tail, and formed entirely of soft rays. Ventral fins absent. Vent situated immediately behind the gill-opening. Air-bladder with a muscular apparatus for dilatation of its anterior part.

A single genus, Fierasfer, with about 10 species, distributed over nearly all the warm and tropical seas, rarely found as far north as the west coast of Ireland. Encheliophis, without pectoral fins, is the larval form of Fierasfer.

Fierasfer spends the greater part of its existence in the interior of Holothurians and other Echinoderms as well as in bivalve Mollusca. It has been observed to enter Holothurians by the posterior or anal aperture, either head first or tail foremost, in the latter case availing itself of the suction which takes place alternately with the expulsion of water by that orifice; it remains near the anus, from which it projects its head in search of food outside its host. It is neither a true parasite nor a commensal or mutualist, in the sense given to these terms by Van Beneden, but simply a lodger, "inquilino," as Emery puts it. Semper, however, regards Encheliophis vermicularis as a true parasite, feeding on the viscera of the Holothurian in which it lives. Putnam has examined eight specimens of a Fierasfer from the Bay of Panama, which were obtained alive from pearl oysters, and also one beautifully enclosed in a pearly covering deposited upon it by the oyster; a similar specimen is preserved in the British Museum.