310 WEST COAST SHELLS 



called species including P. ammon^ 

 based on physiological deformations 

 of this; Fig. 293 is a fairly normal 

 specimen while Fig. 292 is a common 



Fig. 293 ] /• ^• 



deiormation. 



Planorbis opercularis^ Gould, the Operculum-like 

 Planorbis, Plate II, Figs. 8, 12, is a small species 

 with a flat spire, deep narrow umbilicus, and a keel 

 at the edge of the flattened upper surface. In var. 

 planulatus, W. Cooper, PI. II, Fig. 10, the keel is 

 more blunt and medial. \'ar. centervillensis, Tryon, 

 Plate II, Fig. 9, lacks the keel, but the shell can 

 be distinguished from the next species by its deeper 

 whorls and narrow umbilicus. Found in the Colum- 

 bia, Klamath, Utah, Nevada, and Coast Range 

 Systems in streams and lakes. 



Planorbis parvus^ Say, the Dwarf Planorbis, PL 

 II, Fig. 11, is about the same size as the last but 

 is saucer-shaped, has very compressed whorls, and a 

 broad umbilicus. Found among algae and roots in 

 streams and lakes over North America, north oi 

 Mexico. 



{Ajicylida:) 



Ancylus {hanx) patelloides^ Lea, the Mottled 

 River-limpet, PI. II, P'ig. 1, (including L. alt us and 

 A. nezvberryi) is elliptical with a moderately ele- 

 vated, nearly central apex. It is albino, reddish- 

 brown, or with radiating markings within. Lives 

 on rocks and other shells in rapid streams and lakes 

 in the Klamath Svstem. 



Ancylus f rag ills ^ Tryon, the Fragile River-limpet, 



