ELASMOBRANCHIL 



29 



posteriorly along longitudinal surfaces, former not very high 

 and latter shallow. Lower surface of base flat. Length (width) 

 16 mm. 



Fig. 3. — Acrodus humilis Leidy. ' (Type.) I, upper view; 2, lateral view; 

 3, end view; 4, enlarged 4x. 



Formation and locality. I have examined only the above ex- 

 ample, the type ascribed to the "limestone from the New Jersey 

 cretaceous," from Vincentown, in Burlington County [The Vin- 

 centown formation, K]. 



Sub-Order GALEI. 



THE TRUE SHARKS. 



Gill-openings 5, always lateral. Palato-quadrate apparatus 

 not articulated with skull. Dorsal fins 2, well developed, each 

 without spine. 



This group contains the greater number of existing and fossil 

 families of sharks. 



Family GINGLYMOSTOMIDtE. 



THE NURSE SHARKS. 



Eyes very small. Upper and lower lips developed, latter not 

 extending across symphysis. Nasal and buccal cavities confluent. 

 Nasal valves at both sides form one quadrangular flap before 

 mouth, and each provided with a free cylindrical cirrus. Spiracle 



