192 PROCEEDINGS OF THE ACADEMY OF [May-Oct., 



MOLLUSKS FROM LAKE CHAPALA, STATE OP JALISCO AND VICINITY. 



BY HENRY A. PILSBRY. 



Professor Francisco Contreras, in the course of his stadies of 

 the natural resources of Mexico, has made a small collection of the 

 shells of Lake Chapala. The occurrence of pecuUar species of 

 Planorbis, Physa, and Anodoniites probably indicates that there is 

 a considerable endemic element in the mollusk fauna of this Mex- 

 ican lake. 



In treating of the Planorbes and Physas of Lake Patzcuaro', the 

 writer called attention to the more enlarged last whorl of the shell, 

 compared with the most closely related species found in other Mex- 

 ican localities. It was conjectured that larger lung capacity might 

 be advantageous to air-breathing snails of this deep lake. The 

 same peculiarity is seen in Planorbis contrerasi and Physa solidissima 

 of Lake Chapala; but while the shells of Lake Patzcuaro are thin 

 and light, those of Chapala are remarkably solid, perhaps an adap- 

 tation to wave-beaten shores. 



Polygyra ventrosula (Pfr.). 



Chapala. Diameter about 10 mm. Also similar smaller shells, 

 diam. 7.3 mm., which are referable to the variety hmdsvi (Pfr.). 

 Whether these sizes occur in the same colony, or are connected by 

 intermediate sizes, should be noted. 



Polygyra matermontana jaliscoensis Pils. 



Chapala. This form was described from Guadalajara. 

 Drymaeus hegewischi (Pfr.). 



Chapala. 



Planorbis tenuis chapalensis new subspecies. Fig. 1. 



The shell is very strong and solid with narrow, deeply sunken 

 spire on the left side. The aperture is piriform, the penult whorl 

 intruding but little. 



Greatest diam. 16, alt. at aperture 9.3 mm. 



^ These Proceedings for 1891, p. 324. 



