342 CALIFORNIA ACADEMY OF SCIENCES. 



It may be noted here that Mr. Binney, in Land and 

 Freshwater Shells, part 2, p. 127, figures as a variety of 

 P. exacutus the shell called " Pahidina hyalina Lea," in 

 which the spire is raised in a conical form, tending to 

 prove that the species is really dextral. On this account I 

 have described these shells as if in the vertical position in 

 which they are carried by the animal. 



Planorbis (Anisus?) peninsularis n. sp. Plate xiv, 

 fig. 9. 



Shell with both sides qoncave, the right with whorls 

 rounded, their edge forming an obtuse margin, and the 

 outer one partly enclosing the others so that it forms 7^ 

 the greater diameter of shell. Whorls 5, visible on both 

 sides, the rounded (or right) surface showing less of them 

 than the other. Left (or umbilical?) surface nearly flat, 

 deeply concave near middle, the umbilicus being over 

 Y-^ of diameter. Mouth trapezoidal, very oblique, its lips 

 curved, the right extremity attached near the concave 

 spire, the left to the obtuse periphery of shell. Mouth 

 ^ longer than wide; its breadth over )^ of that of shell. 

 Greater diameter 0.16, least 0.05 inch. Color brown, 

 surface smooth. 



Habitat. — With P. anitensis, in same laguna. This shell 

 might, at first sight, pass for the young or a stunted form 

 of the preceding, but it seems to present essential differ- 

 ences too marked to allow of such an inference. Like 

 that it appears to be sinistral but it is impossible to decide 

 which is the umbilical side with certainty. It is also one 

 of the puzzling intermediate forms of the Planorboid 

 group, and belongs as much to Menetus as to Anisus, with 

 some resemblance to Nautilina. It bears much the rela- 

 tion to typical Planorbis that Gonostoma, etc., do lo other 

 Helicoids, and if found on dry ground might easily have 

 been taken for a terrestrial shell. No northern species 

 resembles it much. 



