642 COMPARATIVE ANATOMY 



There are various extinct orders and sub-orders of elasmobranchs, 

 but we shall deal only with two orders and two sub-orders. 



Order, Plagiostomi. 



Sub-Order I. Selachii (12 living and 3 extinct families of sharks 



and dog-fishes, Fig. 367). 

 Sub-Order II. Batoidei (Saw-fishes, skates, rays and torpedoes, 7 



families, Fig. 367). 



Cltimaera monstrosa 



Kain crinacea (common skate) 



Fig. 367. Elasmobranchii. 

 (A, after Goode ; C, after Claus.) 



Order Holocephali (Chimaera, Fig. 367, 1 living and 3 extinct families). 

 The Holocephali are very grotesque looking animals and are of 

 great antiquity. There are peculiar grinding plates in the mouth in- 

 stead of teeth. 



Sub-Class II. Teleostomi. (The true bony fishes). 



Skeleton partly or entirely bony, a single gill-opening on each side 

 leading to gill-arches on which there are gill filaments. There is also 

 a swim-bladder, although this may disappear with age. 



In the higher forms where the skeleton is entirely ossified, the 

 pelvic girdle approaches the pectoral one, so that the pelvic fins may 

 be directly beneath the pectoral fins. It is this approach of the girdles 

 and fins which is used in classifying fish, because this is supposed to 

 show different degrees of specialization. 



The position of the fins in the higher fishes is supposed to furnish 

 evidence to show that amphibians and higher fishes are not closely re- 

 lated. 



Order I. Crossopterygii. 



Sub-Order I. Osteolepida. (4 extinct families). 



Sub-Order II. Cladista. Polypterus and Calamichthys are the 



usual examples. (Fig. 368). 

 Order II. Chondrostei (5 extinct and 2 living families). 



These include the paddle-fishes and sturgeons (Fig. 368). 



