CHORD ATA — VERTEBRATA — FISHES 341 



cereal (Fig. 143). When, with nearly equal lobes, the column 

 stops near the base of the fin, it has received the name homo- 

 cereal (Fig. 149). In most Paleozoic fishes the caudal fin is 

 diphy cereal or heterocercal. From the Cretaceous onward 

 adult bony fish have usually homocercal tail fins, but they are 

 always diphycercal or heterocercal in the immature stage. 



Derivation of name. — Pisces > Latin piseis, a fish. 



The class Pisces is divided into the following sub-classes : 



Page 



1. Elasmobranchii 341 



2. Holocephali 343 



3. Dipneusti 343 



4. Teleostomi 344 



Sub-class I, Elasmobranchii {Sharks, etc.) 



Internal skeleton composed essentially of cartilage. External 

 (dermal) skeleton nearly always present ; when present it is 

 of a placoid type, that is, each scale consists of a basal plate 

 of bony tissue bearing a pointed spine and composed of dentine 

 covered with enamel. Such skin is called shagreen. Dermal 

 fin-rays horny. Cloaca present, serving as the common out- 

 let for the anal, urinary and genital products. Extending out- 

 ward from each gill-arch and supporting the gills is a long, broad 

 plate or septum (whence the name from Greek elasmos, a plate, + 

 Latin branchia, gill) . These fishes breathe by opening and clos- 

 ing the mouth instead of only sucking ; in this they display a 

 vast advance over the Cyclostomata. The sharks lay few eggs, 

 but each contains much yolk, and hence the embryo is well 

 developed when leaving the shell. The majority of the elas- 

 mobranchs are marine, a few genera only living in fresh water. 



The Elasmobranchii (Silurian to present) are sub-divided 

 into the orders : a. Pleuropterygii, — lateral fins very primi- 

 tive as shown in the only well-known genus, Cladoselache, 

 from the Upper Devonian (Cleveland shale) of Ohio ; b. Ichthy- 



