I20 



DAHLGREN. 



[Vol. II. 



oral cavity with their free edges pointing backward, and if the 

 specimen has been hardened they are more easily seen. (See 

 Fig. 2.) 



In fresh dead specimens they are very hard to detect because 

 of their flexible texture and tapering edges, which allow them 



Fig. 2. — Head of flounder, Paralkhthys dentatns (Linn.), seen from left (upper) side. The shaded 

 area represents a vertical median section of the mouth and oral cavity to show the position of 

 the breathing valves in an alcohol hardened specimen. 



to adhere so closely to the mucous membrane of the oral 

 cavity. 



Both valves are j^laced with their edges pointing downward 

 and backward at an angle of less than 45° to the axis of the 

 body. This angle is increased to about 80° when the valves are 

 struck by the regurgitating stream of water at the beginning of 

 expiration. 



