276 SYSTEMATIC ZOOLOGY 



Vertebrata, there is still considerable difference of opinion con- 

 cerning the subdivision of the class itself ; we may divide it into 

 three subclasses. 



SUBCLASS I. ELASMOBRANCHII 



The Elasmobranchii (Gr. iXacrfj.6^, a metal plate, and fipdyxia, 

 gills) are sometimes called the Selachii (Gr. aeXaxos, shark), 

 although this name is perhaps better reserved for the principal 

 order under the subclass. There are two or three extinct 

 orders ; all the living representations of the group are classified 

 under two orders, the one represented by the sharks and the 

 skates and the other by the Chimaera or sea cat. They are 

 almost all marine fishes, and the skin is usually provided with 

 scales, sometimes very numerous ; each scale consists of a bony 

 plate embedded in the skin ; attached to this plate is a sharp 

 spine with its tip pointed backward. The spine is of a very 

 hard bony substance called dentine, and is covered with a thin 

 layer of a still harder substance, the enamel. These scales are 

 called placoid scales, and resemble vertebrate teeth in their ulti- 

 mate structure. Sometimes they are few in number and large, 

 as in some skates ; again, they are very small and arranged close 

 together as in dogfishes; in the latter case the skin of the fish 

 is known as shagreen, and is used in polishing various objects. 



Typically the Elasmobranchii have two dorsal fins, each pro- 

 vided with a sharp, horny spine, a heterocercal caudal fin, one 

 anal tin, and well-developed pelvic and pectoral fins (Fig. 2 v; 

 A caudal tin is sometimes absent. The pelvic fins are placed 

 pretty far back near the anal opening, and in the male are 

 always provided with hooks or claspers, which serve as copu- 

 latory organs. The ova, which are very large, are aim st 

 always fertilized within the body of the female, and in some 

 cases the young are born well-developed, so that such species 

 are viviparous. 



Of the internal anatomy we need note merely that the skeleton 

 is never of bone, but always of cartilage, which, however, is 

 often calcified, in places at lea.st. The notochord is more or 

 less completely replaced by cartilaginous vertebra?, often calci- 

 fied, and the brain is inclosed in a solid cartilaginous box or 

 cranium without sutures or joints. The intestine is of impor- 



