156 



ORDERS OF FISHES. 



series to the Eocene Tertiary ; and Elasmodus is only known 

 from the Eocene. 



SuB-OEDER B. Plagiostomi. — This sub-order is of con- 

 siderably greater importance, as it includes the well-known 

 Sharks and Eays. The vertebral centra are usually more or 

 less ossified, and even when quite cartilaginous, the centra 

 are marked out by distinct rings. The skull is in the form 

 of a cartilaginous capsule, without distinct cranial bones. 

 The mouth is transverse, and is placed on the under surface 

 of the head. 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 

 operculum. 



By Professor Owen tlie Plagiostomi are divided into three 

 sections, termed respectively the Ccstrajjhori, the Sdachii, 

 and the Batides. 





Fig. 519.— Upper jaw of Port Jackson Shark {Cestracion), showing the pavement of crushing 

 teeth. One-half the natural size. (After Owen.) 



a. Ccstrajihori. — In this division there is a strong spine in 

 front of each dorsal fin, and the back teeth are obtuse. The 



