VERTEBRATA : FISHES. 497 



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 Chirncera 

 monstrosa (fig. 269, B), commonly known as the "king of the Herrings." 



Fig. 269. A, Spinax acanthias, one of the Dog-fishes. B, Chim&ra monstrosa. 

 C, Tail-spine of an Eagle-ray (Myliobatis], 



In Chivuzra there is only one apparent gill-slit, but the gills really adhere 

 to the integument by a large portion of their borders, and there are conse- 

 quently five holes communicating with the gill-slit. A rudimentary oper- 

 culum is present, covered by the skin. In the closely-allied Callorhynchtis 

 from the South Seas, there is a large fleshy appendage at the end of the 

 snout. In the Secondary and Tertiary Rocks are found several fossil 

 forms, constituting the genera Edaphodus, Elasmodiis, and Ischiodus. 



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 

 transverse, and is placed on the under surface of the head 

 (fig. 268, A). The exoskeleton consists of placoid granules, 

 tubercles, or spines. The branchial sacs open externally by 

 as many distinct apertures as there are sacs, and there is no 



2 I 



