In the very small sharks called Climatius the fin-spines are very strong, and a series of several free spines occurs, as above stated, on each side between the pectoral and ventral fins, a supposed trace of a former lateral fold. In Paraxus the first dorsal spine is enormously enlarged in size, the other spines remaining much as in Climatius.

Dean on Acanthodei.—In his latest treatise on these fishes, "The Devonian Lamprey," Dr. Dean unites the Pleuropterygii and Acanthodei in a single order under the former name, regarding Acanthoessus as an ally and perhaps descendant of the primitive Cladoselache. Dr. Dean observes:

"In the foregoing classification it will be noted that the Acanthodia are regarded as included under the first order of sharks, Pleuropterygii. To this arrangement Smith Woodward has already objected that the spines of Acanthodians cannot be regarded as the homologues of the radial elements of the Cladoselachian fin (which by a process of concrescence have become fused in its interior margin), since he believes the structure to be entirely dermal in origin. His criticism, however, does not seem to me to be well grounded, for, although all will admit that Acanthodian spines have become incrusted, and deeply incrusted, with a purely dermal calcification, it does not follow that the interior of the spine has not had primitively a non-dermal core. That the concrescence of the radial supporting elements of the fin took place pari passu with the development of a strengthening dermal support of the fin margin was the view expressly formulated in my previous paper on this subject. It is of interest in this connection to recall that the earliest types of Acanthodian spines were the widest, and those which, in spite of their incasing dermal calcification, suggest most clearly the parallel elements representing the component radial supports. There should also be recalled the many features in which the Acanthodians have been shown to resemble Cladoselache."

From these primitive extinct types of shark we may proceed to those forms which have representatives among living fishes. From Cladoselache a fairly direct series extends through the Notidani and Cestraciontes, culminating in the Lamnoid and Galeoid sharks.

Still another series, destitute of anal fin, probably arising near the Acanthodei, reaches its highest development in the side branch of the Batoidei or rays. The Holocephali and Dipneusti must also find their origin in some of these primitive types, certainly not in any form of more highly specialized sharks.

Woodward prefers to place the Tectospondyli next to the Ichthyotomi, leaving the specialized sharks to be treated later. There is, however, no linear system which can interpret natural affinities, and we follow custom in placing the dogfishes and rays at the end of the shark series.

Order Ichthyotomi.—In the order Ichthyotomi (ἰχθύς, fish; τομός, cutting; named by Cope from the supposed segmentation of the cranium; called by Parker and Haswell Pleuracanthea) the very large pectoral fins are developed each as an archipterygium. Each fin consists of a long segmented axis fringed on one or both sides with fin-rays. The notochord is very simple, scarcely or never constricted, the calcifications of its sheath "arrested at the most primitive or rhachitomous stage, except in the tail." This is the best defined of the orders of sharks, and should perhaps rank rather as a subclass, as the Holocephali. Two families of Ichthyotomi are recognized by Woodward, the Pleuracanthidæ and the Cladodontidæ. In the Pleuracanthidæ the dorsal fin is long and low, continuous from head to tail, and the pectoral rays are in two rows. There is a long barbed spine with two rows of serrations at the nape. The body is slender, not depressed, and probably covered with smooth skin. The teeth have two or more blunt cusps, sometimes with a smaller one between and a blunt button behind. The interneural cartilages are more numerous than the neural spines. The genera are imperfectly known, the skeleton of Pleuracanthus decheni only being well preserved. This is the type of the genus called Xenacanthus which, according to Woodward, is identical with Pleuracanthus, a genus otherwise known from spines only. The denticles on the spine are straight or hooked backward, in Pleuracanthus (lævissimus), the spine being flattened. In Orthacanthus (cylindricus), the spine is cylindrical in section. The species called Dittodus and Didymodus are known from the teeth only. These resemble the teeth of Chlamydoselachus. It is not known that Dittodus possesses the nuchal spine, although detached spines like those of Pleuracanthus lie about in remains called Didymodus in the Permian rocks of Texas. In Dicranodus texensis the palato-quadrate articulates with the postorbital process of the cranium, as in the Hexanchidæ, and the hyomandibular is slender.

A genus, Chondrenchelys, from the sub-Carboniferous of Scotland, is supposed to belong to the Pleuracanthidæ, from the resemblance of the skeleton. It has no nuchal spine, and no trace of paired fins is preserved.

The Cladodontidæ differ in having the "pectoral fin developed in the form of a uniserial archipterygium intermediate between the truly biserial one of Pleuracanthus and the pectoral fin of modern sharks." The numerous species are known mainly from detached teeth, especially abundant in America, the earliest being in the Lower Carboniferous. One species, Cladodus nelsoni (Fig. 310), described by Traquair, from the sub-Carboniferous of Scotland shows fairly the structure of the pectoral fin.

In Cladodus mirabilis the teeth are very robust, the crown consisting of a median principal cone and two or three large lateral cones on each side. The cones are fairly striate. In Lambdodus from Illinois there are no lateral cones. Other genera are Dicentrodus, Phœbodus, Carcharopsis, and Hybocladodus.

CHAPTER XXX
THE TRUE SHARKS

Order Notidani.—We may recognize as a distinct order, a primitive group of recent sharks, a group of forms finding its natural place somewhere between the Cladoselachidæ and Heterodontidæ, both of which groups long preceded it in geological time.

The name Notidani (Notidanus, νωτιδάνος, dry back, an old name of one of the genera) may be retained for this group, which corresponds to the Diplospondyli of Hasse, the Opistharthri of Gill, and the Protoselachii of Parker and Haswell. The Notidani are characterized by the primitive structure of the spinal column, which is without calcareous matter, the centra being imperfectly developed. There are six or seven branchial arches, and in the typical forms (not in Chlamydoselachus) the palato-quadrate or upper jaw articulates with the postorbital region of the skull. The teeth are of primitive character, of different forms in the same jaw, each with many cusps. The fins are without spines, the pectoral fin having the three basal cartilages (mesopterygium with propterygium and metapterygium) as usual among sharks.

The few living forms are of high interest. The extinct species are numerous, but not very different from the living species.

Family Hexanchidæ.—The majority of the living Notidanoid sharks belong to the family of Hexanchidæ. These sharks have six or seven gill-openings, one dorsal fin, and a relatively simple organization. The bodies are moderately elongate, not eel-shaped, and the palato-quadrate articulates with the postorbital part of the skull. The six or eight species are found sparsely in the warm seas. The two genera, Hexanchus, with six, and Heptranchias, with seven vertebræ, are found in the Mediterranean. The European species are Hexanchus griseus, the cow-shark, and Heptranchias cinereus. The former crosses to the West Indies. In California, Heptranchias maculatus and Hexanchus corinus are occasionally taken, while Heptranchias deani is the well known Aburazame or oil shark of Japan. Heptranchias indicus, a similar species, is found in India.

Fossil Hexanchidæ exist in large numbers, all of them referred by Woodward to the genus Notidanus (which is a later name than Hexanchus and Heptranchias and intended to include both these genera), differing chiefly in the number of gill-openings, a character not ascertainable in the fossils. None of these, however, appear before Cretaceous time, a fact which may indicate that the simplicity of structure in Hexanchus and Heptranchias is a result of degeneration and not altogether a mark of primitive simplicity. The group is apparently much younger than the Cestraciontes and little older than the Lamnoids, or the Squaloid groups. Heptranchias microdon is common in English Cretaceous rocks, and Heptranchias primigenius and other species are found in the Eocene.

Family Chlamydoselachidæ.—Very great interest is attached to the recent discovery by Samuel Garman of the frilled shark, Chlamydoselachus anguineus, the sole living representative of the Chlamydoselachidæ.

This shark was first found on the coast of Japan, where it is rather common in deep water. It has since been taken off Madeira and off the coast of Norway. It is a long, slender, eel-shaped shark with six gill-openings and the palato-quadrate not articulated to the cranium. The notochord is mainly persistent, in part replaced by feeble cyclospondylic vertebral centra. Each gill-opening is bordered by a broad frill of skin. There is but one dorsal fin. The teeth closely resemble those of Dittodus or Didymodus and other extinct Ichthyotomi. The teeth have broad, backwardly extended bases overlapping, the crown consisting of three slender curved cusps, separated by rudimentary denticles. Teeth of a fossil species, Chlamydoselachus lawleyi, are recorded by J. W. Davis from the Pliocene of Tuscany.

Order Asterospondyli.—The order of Asterospondyli comprises the typical sharks, those in which the individual vertebræ are well developed, the calcareous lamellæ arranged so as to radiate, star-fashion, from the central axis. All these sharks possess two dorsal fins and one anal fin, the pectoral fin is normally developed, with the three basal cartilages; there are five gill-openings, and the tail is heterocercal.

Suborder Cestraciontes.—The most ancient types may be set off as a distinct suborder under the name of Cestraciontes or Prosarthri.

These forms find their nearest allies in the Notidani, which they resemble to some extent in dentition and in having the palato-quadrate articulated to the skull although fastened farther forward than in the Notidani. Each of the two dorsal fins has a strong spine.

Family Heterodontidæ.—Among recent species this group contains only the family of Heterodontidæ, the bullhead sharks, or Port Jackson sharks. In this family the head is high, with usually projecting eyebrows, the lateral teeth are pad-like, ridged or rounded, arranged in many rows, different from the pointed anterior teeth, the fins are large, the coloration is strongly marked, and the large egg-cases are spirally twisted. All have five gill-openings. The living species of Heterodontidæ are found only in the Pacific, the Port Jackson shark of Australia, Heterodontus philippi, being longest known. Other species are Heterodontus francisci, common in California, Heterodontus japonicus, in Japan, and Heterodontus zebra, in China. These small and harmless sharks at once attract attention by their peculiar forms. In the American species the jaws are less contracted than in the Asiatic species, called Heterodontus. For this reason Dr. Gill has separated the former under the name of Gyropleurodus. The differences are, however, of slight value. The genus Heterodontus first appears in the Jurassic, where a number of species are known, one of the earliest being Heterodontus falcifer.

Three families of Cestraciontes are recognized by Hay. The most primitive of these is the group of Orodontidæ. Orodus, from the Lower Carboniferous, has the teeth with a central crown, its surface wrinkled. Of the Heterodontidæ, Hybodus, of the Carboniferous and Triassic, is one of the earliest and largest genera, characterized by elongate teeth of many cusps, different in different parts of the jaw, somewhat as in the Hexanchidæ, the median points being, however, always longest. The dorsal fins are provided with long spines serrated behind. The vertebræ with persistent notochord show qualities intermediate between those of Hexanchidæ and Heterodontidæ, and the same relation is shown by the teeth. In this genus two large hooked half-barbed dermal spines occur behind each orbit.

Palæospinax, with short stout spines and very large pectoral fins, formerly regarded as a dogfish, is placed near Heterodontus by Woodward. Acrodus, from the Triassic, shows considerable resemblance to Heterodontus. Its teeth are rounded and without cusps.

Most of these species belong to the Carboniferous, Triassic, and Jurassic, although some fragments ascribed to Cestraciont sharks occur in the Upper Silurian. Asteracanthus, known only from fin-spines in the Jura, probably belongs here.

It is a singular fact first noted by Dr. Hay, that with all the great variety of sharks, ten families in the Carboniferous age, representatives of but one family, Heterodontidæ, are found in the Triassic. This family may be the parent of all subsequent sharks and rays, six families of these appearing in the Jurassic and many more in the Cretaceous.

Edestus and its Allies.—Certain monstrous structures, hitherto thought to be fin-spines, are now shown by Dr. Eastman and others to be coalescent teeth of Cestraciont sharks.

These remarkable Ichthyodorulites are characteristic structures of sharks of unknown nature, but probably related to the Heterodontidæ. Of these the principal genera are Edestus, Helicoprion, and Campyloprion. Karpinsky regards these ornate serrated spiral structures as whorls of unshed teeth cemented together and extending outside the mouth, "sharp, piercing teeth which were never shed but became fused in whorls as the animals grew."

Dr. Eastman has, however, shown that these supposed teeth of Edestus are much like those of the Cochliodontidæ, and the animals which bore them should doubtless find their place among the Cestraciont sharks, perhaps within the family of Heterodontidæ.

Onchus.—The name Onchus was applied by Agassiz to small laterally compressed spines, their sides ornamented with smooth or faintly crenulated longitudinal ridges, and with no denticles behind. Very likely these belonged to extinct Cestraciont sharks. Onchus murchisoni and Onchus tenuistriatus occur in the Upper Silurian rocks of England, in the lowest strata in which sharks have been found.

To a hypothetical group of primitive sharks Dr. Hasse has given the name of Polyospondyli. In these supposed ancestral sharks the vertebræ were without any ossification, a simple notochord, possibly swollen at intervals. The dorsal fin was single and long, a fold of skin with perhaps a single spine as an anterior support. The teeth must have been modified dermal papillæ, each probably with many cusps. Probably seven gill-openings were developed, and the tail was diphycercal, ending in a straight point. The finely striated fin-spines not curved upward at tip, called Onchus from the Upper Silurian of the Ludlow shales of England and elsewhere, are placed by Hasse near his Polyspondylous sharks. Such spines have been retained by the group of Chimæras, supposed to be derived from the ancestors of Onchus, as well as by the Heterodontidæ and Squalidæ.

Family Cochliodontidæ.—Another ancient family known from teeth alone is that of Cochliodontidæ. These teeth resemble those of the Heterodontidæ, but are more highly specialized. The form of the body is unknown, and the animals may have been rays rather than sharks. Eastman leaves them near the Petalodontidæ, which group of supposed rays shows a similar dentition. The teeth are convex in form, strongly arched, hollowed at base, and often marked by ridges or folds, being without sharp cusps. In each jaw is a strong posterior tooth with smaller teeth about. The elaborate specialization of these ancient teeth for crushing or grinding shells is very remarkable. The species are chiefly confined to rocks of the Carboniferous age. Among the principal genera are Helodus, Psephodus, Sandalodus, Venustodus, Xystrodus, Deltodus, Pœcilodus, and Cochliodus.

Concerning the teeth of various fossil sharks, Dr. Dean observes: "Their general character appears to have been primitive, but in structural details they were certainly specialized. Thus their dentition had become adapted to a shellfish diet, and they had evolved defensive spines at the fin margins, sometimes at the sides of the head. In some cases the teeth remain as primitive shagreen cusps on the rim of the mouth, but become heavy and bluntish behind; in other forms the fusion of tooth clusters may present the widest range in their adaptations for crushing; and the curves and twistings of the tritoral surfaces may have resulted in the most specialized forms of dentition which are known to occur, not merely in sharks but among all vertebrates."

In this neighborhood belongs, perhaps, the family of Tamiobatidæ, known from the skull of a single specimen, called Tamiobatis vetustus, from the Devonian in eastern Kentucky. The head has the depressed form of a ray, but it is probably a shark and one of the very earliest known.

Suborder Galei.—The great body of recent sharks belong to the suborder Galei, or Euselachii, characterized by the asterospondylous vertebræ, each having a star-shaped nucleus, and by the fact that the palato-quadrate apparatus or upper jaw is not articulated with the skull. The sharks of this suborder are the most highly specialized of the group, the strongest and largest and, in general, the most active and voracious. They are of three types and naturally group themselves about the three central families Scyliorhinidæ, Lamnidæ, and Carchariidæ (Galeorhinidæ).

The Asterospondyli are less ancient than the preceding groups, but the modern families were well differentiated in Mesozoic times.

Among the Galei the dentition is less complex than with the ancient forms, although the individual teeth are more highly specialized. The teeth are usually adapted for biting, often with knife-like or serrated edges; only the outer teeth are in function; as they are gradually lost, the inner teeth are moved outward, gradually taking the place of these.

We may place first, as most primitive, the forms without nictitating membrane.

Family Scyliorhinidæ.—The most primitive of the modern families is doubtless that of the Scyliorhinidæ, or cat-sharks. This group includes sharks with the dorsal fins both behind the ventrals, the tail not keeled and not bent upward, the spiracles present, and the teeth small and close-set. The species are small and mostly spotted, found in the warm seas. All of them lay their eggs in large cases, oblong, and with long filaments or strings at the corners. The cat-sharks, or roussettes, Scyliorhinus canicula and Catulus stellaris, abound in the Mediterranean. Their skin is used as shagreen or sandpaper in polishing furniture. The species of swell-sharks (Cephaloscylium) (C. uter, in California; C. ventriosus, in Chile; C. laticeps, in Australia; C. umbratile, in Japan) are short, wide-bodied sharks, which have the habit of filling the capacious stomach with air, then floating belly upward like a globefish. Other species are found in the depths of the sea. Scyliorhinus, Catulus, and numerous other genera are found fossil. The earliest is Palæoscyllium, in the Jurassic, not very different from Scyliorhinus, but the fins are described as more nearly like those of Ginglymostoma.

Close to the Scyliorhinidæ is the Asiatic family, Hemiscylliidæ, which differs in being ovoviviparous, the young, according to Mr. Edgar R. Waite, hatched within the body. The general appearance is that of the Scyliorhinidæ, the body being elongate. Chiloscyllium is a well-known genus with several species in the East Indies. Chiloscyllium modestum is the dogfish of the Australian fishermen. The Orectolobidæ are thick-set sharks, with large heads provided with fleshy fringes. Orectolobus barbatus (Crossorhinus of authors) abounds from Japan to Australia.

Another family, Ginglymostomidæ, differs mainly in the form of the tail, which is long and bent abruptly upward at its base. These large sharks, known as nurse-sharks, are found in the warm seas. Ginglymostoma cirrhatum is the common species with Orectolobus. Stegostoma tigrinum, of the Indian seas and north to Japan, one of several genera called tiger-sharks, is remarkable for its handsome spotted coloration. The extinct genus Pseudogaleus (voltai) is said to connect the Scyliorhinoid with the Carcharioid sharks.

The Lamnoid or Mackerel Sharks.—The most active and most ferocious of the sharks, as well as the largest and some of the most sluggish, belong to a group of families known collectively as Lamnoid, because of a general resemblance to the mackerel-shark, or Lamna, as distinguished from the blue sharks and white sharks allied to Carcharias (Carcharhinus).

The Lamnoid sharks agree with the cat-sharks in the absence of nictitating membrane or third eyelid, but differ in the anterior insertion of the first dorsal fin, which is before the ventrals. Some of these sharks have the most highly specialized teeth to be found among fishes, most effective as knives or as scissors. Still others have the most highly specialized tails, either long and flail-like, or short, broad, and muscular, fitting the animal for swifter progression than is possible for any other sharks. The Lamnoid families are especially numerous as fossils, their teeth abounding in all suitable rock deposits from Mesozoic times till now. Among the Lamnoid sharks numerous families must be recognized.

The most primitive is perhaps that of the Odontaspididæ (called Carchariidæ by some recent authors), now chiefly extinct, with the tail unequal and not keeled, and the teeth slender and sharp, often with smaller cusps at their base. Odontaspis and its relatives of the same genus are numerous, from the Cretaceous onward, and three species are still extant, small sharks of a voracious habit, living on sandy shores. Odontaspis littoralis (also known as Carcharias littoralis) is the common sand-shark of our Atlantic coast. Odontaspis taurus is a similar form in the Mediterranean.

Family Mitsukurinidæ, the Goblin-sharks.—Closely allied to Odontaspis is the small family of Mitsukurinidæ, of which a single living species is known. The teeth are like those of Odontaspis, but the appearance is very different.

The goblin-shark, or Tenguzame, Mitsukurina owstoni, is a very large shark rarely taken in the Kuro Shiwo, or warm "Black Current" of Japan. It is characterized by the development of the snout into a long flat blade, extending far beyond the mouth, much as in Polyodon and in certain Chimæras. Several specimens are now known, all taken by Capt. Alan Owston of Yokohama in Sagami Bay, Japan. The original specimen, a young shark just born, was presented by him to Professor Kakichi Mitsukuri of the University of Tokyo. From this our figure was taken. The largest specimen now known is in the United States National Museum and is fourteen feet in length. In the Upper Cretaceous is a very similar genus, Scapanorhynchus (lewisi, etc.), which Professor Woodward thinks may be even generically identical with Mitsukurina, though there is considerable difference in the form of the still longer rostral plate, and the species of Scapanorhynchus differ among themselves in this regard.

Mitsukurina, with Heterodontus, Heptranchias, and Chlamydoselache, is a very remarkable survival of a very ancient form. It is an interesting fact that the center of abundance of all these relics of ancient life is in the Black Current, or Gulf Stream, of Japan.

Family Alopiidæ, or Thresher Sharks.—The related family of Alopiidæ contains probably but one recent species, the great fox-shark, or thresher, found in all warm seas. In this species, Alopias vulpes, the tail is as long as the rest of the body and bent upward from the base. The snout is very short, and the teeth are small and close-set. The species reaches a length of about twenty-five feet. It is not especially ferocious, and the current stories of its attacks on whales probably arise from a mistake of the observers, who have taken the great killer, Orca, for a shark. The killer is a mammal, allied to the porpoise. It attacks the whale with great ferocity, clinging to its flesh by its strong teeth. The whale rolls over and over, throwing the killer into the air, and sailors report it as a thresher. As a matter of fact the thresher very rarely if ever attacks any animal except small fish. It is said to use its tail in rounding up and destroying schools of herring and sardines. Fossil teeth of thresher-sharks of some species are found from the Miocene.

Family Pseudotriakidæ.—The Pseudotriakidæ consist of two species. One of these is Pseudotriakis microdon, a large shark with a long low tail, long and low dorsal fin, and small teeth. It has been only twice taken, off Portugal and off Long Island. The other, the mute shark, Pseudotriakis acrales, a large shark with the body as soft as a rag, is in the museum of Stanford University, having been taken by Mr. Owston off Misaki.

Family Lamnidæ.—To the family of Lamnidæ proper belong the swiftest, strongest, and most voracious of all sharks. The chief distinction lies in the lunate tail, which has a keel on either side at base, as in the mackerels. This form is especially favorable for swift swimming, and it has been independently developed in the mackerel-sharks, as in the mackerels, in the interest of speed in movement.

The porbeagle, Lamna cornubica, known as salmon-shark in Alaska, has long been noted for its murderous voracity. About Kadiak Island it destroys schools of salmon, and along the coasts of Japan, and especially of Europe and across to New England, it makes its evil presence felt among the fishermen. Numerous fossil species of Lamna occur, known by the long knife-like flexuous teeth, each having one or two small cusps at its base.

In the closely related genus, Isurus, the mackerel-sharks, this cusp is wanting, while in Isuropsis the dorsal fin is set farther back. In each of these genera the species reach a length of 20 to 25 feet. Each is strong, swift, and voracious. Isurus oxyrhynchus occurs in the Mediterranean, Isuropsis dekayi, in the Gulf of Mexico, and Isuropsis glauca, from Hawaii and Japan westward to the Red Sea.

Man-eating Sharks.—Equally swift and vastly stronger than these mackerel-sharks is the man-eater, or great white shark, Carcharodon carcharias. This shark, found occasionally in all warm seas, reaches a length of over thirty feet and has been known to devour men. According to Linnæus, it is the animal which swallowed the prophet Jonah. "Jonam Prophetum," he observes, "ut veteris Herculem trinoctem, in hujus ventriculo tridui spateo bæsisse, verosimile est."

It is beyond comparison the most voracious of fish-like animals. Near Soquel, California, the writer obtained a specimen in 1880, with a young sea-lion (Zalophus) in its stomach. It has been taken on the coasts of Europe, New England, Carolina, California, Hawaii, and Japan, its distribution evidently girdling the globe. The genus Carcharodon is known at once by its broad, evenly triangular, knife-like teeth, with finely serrated edges, and without notch or cusp of any kind. But one species is now living. Fossil teeth are found from the Eocene. One of these, Carcharodon megalodon (Fig. 332), from fish-guano deposits in South Carolina and elsewhere, has teeth nearly six inches long. The animal could not have been less than ninety feet in length. These huge sharks can be but recently extinct, as their teeth have been dredged from the sea-bottom by the Challenger in the mid-Pacific.

Fossil teeth of Lamna and Isurus as well as of Carcharodon are found in great abundance in Cretaceous and Tertiary rocks. Among the earlier species are forms which connect these genera very closely.

The fossil genus Otodus must belong to the Lamnidæ. Its massive teeth with entire edges and blunt cusps at base are common in Cretaceous and Tertiary deposits. The teeth are formed much as in Lamna, but are blunter, heavier, and much less effective as instruments of destruction. The extinct genus Corax is also placed here by Woodward.

Family Cetorhinidæ, or Basking Sharks.—The largest of all living sharks is the great basking shark (Cetorhinus maximus), constituting the family of Cetorhinidæ. This is the largest of all fishes, reaching a length of thirty-six feet and an enormous weight. It is a dull and sluggish animal of the northern seas, almost as inert as a sawlog, often floating slowly southward in pairs in the spring and caught occasionally by whalers for its liver. When caught, its huge flabby head spreads out wide on the ground, its weight in connection with the great size of the mouth-cavity rendering it shapeless. Although so clumsy and without spirit, it is said that a blow with its tail will crush an ordinary whaleboat. The basking shark is known on all northern coasts, but has most frequently been taken in the North Sea, and about Monterey Bay in California. From this locality specimens have been sent to the chief museums of Europe. In its external characters the basking shark has much in common with the man-eater. Its body is, however, relatively clumsy forward; its fins are lower, and its gill-openings are much broader, almost meeting under the throat. The great difference lies in the teeth, which in Cetorhinus are very small and weak, about 200 in each row. The basking shark, also called elephant-shark and bone-shark, does not pursue its prey, but feeds on small creatures to be taken without effort. Fossil teeth of Cetorhinus have been found from the Cretaceous, as also fossil gill-rakers, structures which in this shark are so long as to suggest whalebone.

Family Rhineodontidæ.—The whale-sharks, Rhineodontidæ, are likewise sluggish monsters with feeble teeth and keeled tails. From Cetorhinus they differ mainly in having the last gill-opening above the pectorals. There is probably but one species, Rhineodon typicus, of the tropical Pacific, straying northward to Florida, Lower California, and Japan.

The Carcharioid Sharks, or Requins.—The largest family of recent sharks is that of Carchariidæ (often called Galeorhinidæ, or Galeidæ), a modern offshoot from the Lamnoid type, and especially characterized by the presence of a third eyelid, the nictitating membrane, which can be drawn across the eye from below. The heterocercal tail has no keel; the end is bent upward; both dorsal fins are present, and the first is well in front of the ventral fins; the last gill-opening over the base of the pectoral, the head normally formed; these sharks are ovoviviparous, the young being hatched in a sort of uterus, with or without placental attachment.

Some of these sharks are small, blunt-toothed, and innocuous. Others reach a very large size and are surpassed in voracity only by the various Lamnidæ.

The genera Cynias and Mustelus, comprising the soft-mouthed or hound-sharks, have the teeth flat and paved, while well-developed spiracles are present. These small, harmless sharks abound on almost all coasts in warm regions, and are largely used as food by those who do not object to the harsh odor of shark's flesh. The best-known species is Cynias canis of the Atlantic. By a regular gradation of intermediate forms, through such genera as Rhinotriacis and Triakis with tricuspid teeth, we reach the large sharp-toothed members of this family. Galeus (or Galeorhinus) includes large sharks having spiracles, no pit at the root of the tail, and with large, coarsely serrated teeth. One species, the soup-fin shark (Galeus zyopterus), is found on the coast of California, where its fins are highly valued by the Chinese, selling at from one to two dollars for each set. The delicate fin-rays are the part used, these dissolving into a finely flavored gelatine. The liver of this and other species is used in making a coarse oil, like that taken from the dogfish. Other species of Galeus are found in other regions, Galeus galeus being known in England as tope, Galeus japonicus abounding in Japan.

Galeocerdo differs mainly in having a pit at the root of the tail. Its species, large, voracious, and tiger-spotted, are found in warm seas and known as tiger-sharks (Galeocerdo maculatus in the Atlantic, Galeocerdo tigrinus in the Pacific).

The species of Carcharias (Carcharhinus of Blainville) lack the spiracles. These species are very numerous, voracious, armed with sharp teeth, broad or narrow, and finely serrated on both edges. Some of these sharks reach a length of thirty feet. They are very destructive to other fishes, and often to fishery apparatus as well. They are sometimes sought as food, more often for the oil in their livers, but, as a rule, they are rarely caught except as a measure for getting rid of them. Of the many species the best known is the broad-headed Carcharias lamia, or cub-shark, of the Atlantic. This the writer has taken with a great hook and chain from the wharves at Key West. These great sharks swim about harbors in the tropics, acting as scavengers and occasionally seizing arm or leg of those who venture within their reach. One species (Carcharias nicaraguensis) is found in Lake Nicaragua, the only fresh-water shark known, although some run up the brackish mouth of the Ganges and into Lake Pontchartrain. Carcharias japonicus abounds in Japan.