ELASMOBRANCHII. 



421 



a large portion of their borders, and there are consequently 

 five holes communicating with the gill-slit. A rudimentary 

 operculum is present, covered by the skin. In the closely- 

 allied Callorhynchus 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, Elasmodus, and Ischiodus. 



Fig. 179. Plagiostomi and Holocephali. A, White Shark (Carcharias) ; 

 B, Chimtera monstrosa. (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 

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

 (fig. 178, 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 oper- 

 culum. A pair of tubes proceed from the pharynx to open 

 on the upper surface of the head by two apertures, which are 



