372 



MANUAL OF ZOOLOGY. 



teeth. The exoskeleton consists of placoid granules. The 

 first ray of the anterior dorsal fin is in the form of a powerful 

 defensive spine, like the " ichthyodorulites " of many fossil 

 fishes. The ventral fins are abdominal, and the tail is hetero- 

 cercal. There is only a single external gill-aperture, covered 

 with a gill-cover and branchiostegal membrane ; but only a 

 small portion of the borders of the branchial laminae is free. 

 The mouth is placed at the extremity of the head. 



The best-known living representative of the sub-order is the 

 Chimcera monstrosa (fig. 142, B), commonly known as the 

 " king of the Herrings." In the Secondary and Tertiary 

 Rocks, however, are found several fossil forms, constituting 

 the genera Edaphodus, Elasmodus, and Ischiodus. 



Fig. 142. Plagiostomi and Holocephali. A, White Shark (Carcharias); 

 B, Chimtzra motistrosa. (After Gosse). 



SUB-ORDER B. PLAGIOSTOMI. This sub-order is of consider- 

 ably greater importance, as it includes the well-known Sharks 

 and Rays. The vertebral centra are usually more or less ossi- 

 fied, and even when quite cartilaginous, the centra are marked 

 out by distinct rings. The skull is in the form of a cartila- 

 ginous capsule, without distinct cranial bones. The mouth is 



