CLASSIFICATION.] ICHTHYOLOGY 685 spread, deep-sea types. Eels are well represented, and seem to descend to the greatest depths. Myxine has been ob tained from a depth of 345 fathoms. The greatest depth hitherto reached by a dredge in which fishes were enclosed is 2900 fathoms. But the specimens thus obtained belong to a species (Gonostoma microdon) which seems to be extremely abundant in upper strata of the Atlantic and Pacific, and were therefore most likely caught by the dredge in its ascent. The next greatest depth, viz., 2750 fathoms, must be accepted as one at which fishes do undoubtedly live, the fish obtained from this depth of the Atlantic, Bathyophls ferox, showing by its whole habit that it is a form living on the bottom of the ocean. CLASSIFICATION. The class of fishes is divided into four subclasses : I. Pala ichthyes. Heart with a contractile conus arte- riosus ; intestine with a spiral valve ; optic nerves non- decussating, or only partially decussating. II. Teleostei. Heart with a non-contractile bulbus arteriosus ; intestine without spiral valve ; optic nerves decussating. Skeleton ossified, with completely separated vertebrae. III. Cyclostornata. Heart without bulbus arteriosus ; intestine simple. Skeleton cartilaginous and notochordal. One nasal aperture only. No jaws ; mouth surrounded by a circular lip. IV. Leptocardii. Heart replaced by pulsating sinuses ; intestine simple. Skeleton membrano-cartilaginous and notochordal. No brain; no skull. Subclass I, Palceickthyes. This subclass comprises the sharks and rays and the Ganoid fishes. Though it is based upon a singular con currence of most important characters, its members exhibit as great a diversity of form, and as manifold modifications in the remainder of their organization, as the Tdeostei. The Paloeichthyes stand to the Teleostei in the same relation as the Marsupials to the Plar,entalia. Geologically, as a sub class, they were the predecessors of Teleosteous fishes ; and it is a remarkable fact that all those modifications which show an approach of the ichthyic type to the Batrachians are found in this subclass. It is divided into two orders, the Chondropterygii and the Ganoidei. ORDER I. Chondroptcryyii. Skeleton cartilaginous. Body with medial and paired fins, the hinder pair abdominal. Vertebral column generally heterocercal, the upper lobe of the caudal fin produced. Gills attached to the skin by the outer margin, with several intervening gill-openings ; n rely one external gill-opening only. No gill-cover. No air- i ladder. Two, three, or more series of valves in the conus arteriosus. Ova large and few in number, impregnated and, in some species, developed within a uterine cavity. Embryo with deciduous external gills. Males with intromittent organs attached to the ventral fins. Tliis order, for which, also, the name Elasmobranchii has been froposcd (by Bonaparte), comprises the .sharks, rays, and chimseras. t is divided into two suborders, Plagiostomata and Holoccpluila. SUBORDER I. Plngiodomntn. From five to seven gill-openings. Skull with a suspensorium and the palatal apparatus detached. Teeth numerous. The Plagiostomes differ greatly from each other with regard to the general form of the body. In the sharks, or Sclachoidei, the body is elongate, more or less cylindrical, gradually passing into the tail ; the gill-openings are lateral. In the rays, or Batoidri, the gill-openings are always placed on the abdominal aspect of the fish ; the body is depressed, and the trunk, win is surrounded by the_ immensely developed pectoral fins, forms abroad flat disk, of which the tail appears as a thin and slender appendage. Spiracles are always present ; the number of gill-openings is constantly five ; there is no anal fin ; the dorsal fins, if present, are situated on the tail. Some of the rays, however, approach the sharks in having th-i "audal portion less abruptly contracted behind the trunk. Fossil Pligiostomes are very numerous in all formations. Some of the earliest determinable fish remains are, or are believed to be, derived from PIngiostomes. Those which can be referred to any of the families specified l>elow will be mentioned in due course; but there are others, especially fin-spines, which leave us in doubt to which group of Plugiuitouies their owners had affinity: as Onckus, from the Upper Silurian, continuing to Carboniferous formations; Dimcracanthus, Homaca.nthus, from the Devonian ; Oracnnthus, Gyracanthus, Tristychius, Astroplychius, Ptychacanthus, Sphcna- cantkus, &c., from Carboniferous formations; Lcptacanthus, from the Coal to the Oolite ; (Jladacanthus, Oricacanthus, Gyro2>ristis, and Lepracnnthus, from the Coal-measures ; Nemacanthus, Liacanthus, from the Trias ; Astracanthus, Myriacanthus, Pristacanthus from the Jurassic group. A. Sclachoidei, Sharks. Family 1. Carchariidce. Eye with a nictitating membrane. Mouth crescent-shaped, inferior. Anal fin present. Two dorsal fins, the first opposite to the space between the pectoral and ventral fins, without spine in front. Genera: Carcharias (Blue Shark), Galcocerdo, Galeus (Tope), Zygccna (Hammerhead), Mustclus (Hound), HemicjaUus, Loxodon, Thalassorhinus, Triocnodon, Lcptocarcharias, and Triads. Fossil : Corax and Ifemipristis. Family 2. Lamnidce. Eye without nictitating membrane. Anal fin present. Two dorsal fins, the first opposite to the space between the pectoral and ventral fins, without spine in front. Nostrils not confluent with the mouth, which is inferior. Spiracles absent or minute. Genera: Lamna (Porbeagle), Car char odon, Odontaspis, Alopecias (Fox-Shark), and Selachc (Basking Shark). Fossil : Carcharopsis, Oxyl.cn, Sphenodus, GompJwdus, and Ancistrodon. Family 3. Rhinodontidce. No nictitating membrane. Anal fin present. Two dorsal fins, the first nearly opposite to the ventrals, without spine in front. Mouth and nostril near the extremity of the snout. This small family comprises one species only, Rkinodon typicus, a gigantic shark, which is known to exceed 50 feet in length. Family 4. Notidanidce. No nictitating membrane. One dorsal fin only, without spine, opposite to the anal. Dentition unequal in the jaws ; in the upper jaw one or two pairs of awl-shaped teeth, the following six being broader, and provided with several cusps, one of which is much stronger than the others. Lower jaw with six large comb-like teeth on each side, besides the smaller posterior teeth. Gill-openings wide, six in number in Hexanchus, seven in Heptanchus. Fossil teeth belonging to this type have been found in Jurassic and later formations (Notidanus and JEllopos). Family 5. Scylliidce. Two dorsal fins, without spine, the first above or behind the ventrals ; anal fin present. No nictitating membrane. Spiracle always distinct. Mouth inferior. Teeth small, several series generally being in function. Genera: Scyllium (Dog- Fishes), Pristiurus, Parascyllium, Gin< glymostoma, Stcgostoma, Chiloscyllium, Crossorhinus. Fossil : Scyl- liodus, Palccoscyllium, ThyeUina. Family 6. Hybodontidoc. Two dorsal fins, each with a serrated spine. Teeth rounded, longitudinally striated, with one larger and from two to four smaller lateral cusps. Skin covered with shagreen. Extinct. From Carboniferous, Liassic, and Triassic formations. Several genera have been distinguished ; and, if Cladodus belongs to this family, it was represented even in the Devonian. Family 7. Cestraciontidce. No nictitating membrane. Two dorsal fins, the first opposite to the space between the pectoral and ventral fins ; anal fin present. Nasal and buccal cavities confluent, Teeth obtuse, several series being in function. This family is one of particular interest, because representatives of it occur in numerous modifications in Primary and Secondary strata. Their dentition is uniformly adapted for the prehension and mastication of crustaceous and hard-shelled animals. The fossil forms far exceeded in size the species of the only surviving genus, Ccstracion (the Port Jackson Shark) ; they make their appearance with Ctcnoptychius in the Devonian ; this is succeeded in the Coal- measures by Psammodus, Chomatodus, Pctrodvs, Cochliodns, Poly- rhizodits, &c., and in the Trias and Chalk by Strophodus, Acmdus, Thectodus, and Ptychodus. Of the 25 genera known, 22 have lived in the periods preceding the Oolitic. Family 8. Spinacidcc. No membrana nictitans. Two dorsal fins ; no anal. Mouth but slightly arched ; a long, deep, straight, oblique groove on each side of the mouth. Spiracles present ; gill- openings narrow. Pectoral fins not notched at their origin. Genera : Ccntrina, Acanthias (Spiny Dog-Fish), Ccntrophorus, Spi tutx, Centroscyllium, Scymnus, Lcemanji ^ (Greenland Shark), Echinorhinus, Euprotomicrus and Isistius. The largest of thrse fishes is the Greenland shark, which attains to a length of 15 feet, and is common in the Arctic regions. Fossil genera : Palccosjnnax and Drrpancphoru.i, from Cretaceous and Jurassic formations. Family 9. Rhinidce. No anal fin ; two dorsal fins. Spiracles
present. Pectoral fins large, with the basal portion prolonged
