416 ZOOLOGY. 



column or series of biconcave vertebrae, with the cartilage in 

 part replaced by bone, forming radiating leaves or plates ; 

 while in the rays or skates the anterior part of the column 

 is bony. 



The ribs are small, sometimes rudimentary. The skull is 

 rudimentary, without membrane-bones, embryonic in char- 

 acter, forming a simple cartilaginous brain-box, without pre- 

 maxillary or maxillary bones, the constitution of the jaws be- 

 ing quite unlike that of the bony fishes, the jaws being formed 

 of elements, i. e., "cartilaginous representatives of the pri- 

 mary palatoquadrate arch and of Meckel's cartilage." (Hux- 



ley.) 



There arc no opercular bones such as cover the gill-open- 

 ings in bony fishes, their place being taken by cartilaginous 

 filaments. 



The mouth is armed in most sharks with numerous sharp, 

 flattened, conical teeth, arranged in transverse rows and 

 pointing backwards ; they are never fixed in sockets, but 

 imbedded in the mucous membrane of the upper and under 

 jaws. In the Heterodontidae, represented by Cestracion or 

 Port Jackson shark, the teeth are much blunter than in 

 other living sharks, the middle and hinder teeth having 

 broad, flattened crowns, forming a pavement of rounded 

 teeth. The Devonian sharks were in most cases like the 

 Cestracion in this respect. In the Carboniferous age, sharks 

 with teeth more like those of modern forms came into ex- 

 istence ; and they must have been of a more active nature, 

 the sharp teeth directed backwaixl indicating the rapacity of 

 these monsters, which darting after and seizing their prey 

 were enabled to retain it by the backward-pointed teeth ; 

 while the more sluggish Devonian Cestracions kept near the 

 bottom and devoured the shelled mollusks, etc., possibly Or- 

 thoceratites, Nautili, and Trilobites, which became nearly 

 extinct about the time the type of pavement-toothed sharks 

 culminated. 



The teeth of the skates or rays have obtuse points. In 

 Myliobatis the teeth are flattened and united to form a solid 

 pavement, so that the mouths of these large rays are fur- 



