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 Teeth of 

 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 



