FISHES 



343 



flattened-out sharks the slitlike gill openings 

 are five to seven on each side, the skeleton is 



From "Animate Creation 



FIG. 126. A dogfish, a small species of shark, with two of its young 

 and two of its egg cases. 



cartilaginous, and the skin is beset with thorn- Piacoid 

 like (placoid) scales, or granules, but in Mus- 

 telus, the dog shark, with pointed, overlapping 

 scales. The eggs are large and comparatively 

 few ; they are deposited in leatherlike cases or 

 hatched within the body. The teeth of sharks 

 are characteristic usually pointed or more or 

 less serrated, often triangular, and sometimes 

 very large. They are extremely hard, and con- 

 sequently are often preserved in the rocks as 

 fossils. Associated with the elasmobranchs, 

 but very peculiar, are the Holocephali or Chi- 

 mseras, which are comparatively rare today, but 



Teeth ol 

 sharks 



