=Limnæa=, Lam. Subgenera: _Limnæa_, Type _L. stagnalis_, Lin. _Limnophysa_, Fitz., Type _L. palustris_, Müll. =Physa=, Drap. Subgenera: _Physa_, Type _P. heterostropha_, Say. _Bulinus_, H. & A. Ads., Type _B. hypnorum_, Lin. _Subfamily_ PLANORBINÆ. W. C. Genera: =Planorbis=, Guett. Subgenera: _Planorbis_, Type _P. subcrenatus_, Cpr. _Helisoma_, Sw., Type _H. ammon_, Gld. _Menetus_, H. & A. Ads., Type _M. opercularis_, Gld. =Carinifex=, W. G. Binney. Type _C. Newberryi_, Lea. _Subfamily_ ANCYLINÆ. W. C. Genera: =Ancylus=, Geoffr. Type _A. Newberryi_, Lea. =Acroloxus=, Beck. Type _A. Nuttallii_, Lea. =Gundlachia=, Pfr. Type G. _Californica_, Rowell. _Subfamily_ POMPHOLINÆ, Dall. _Testa depressa, paucis spiralis, anf. ult. maxima. Columella recta, sine plica._ _Animalis tentaculis longis, oculiferis, et alius par oculorum, in bases interiores tentaculorum._ Characters, _Shell_; depressed, few whorled, last whorl the largest, without fold on the columella. Inoperculate. _Soft parts_; foot rounded, tentacles long, bearing eyes; another pair of eyes situated on the inner bases of the tentacles. Fluviatile, phytophagous. Genus =Pompholyx=, Lea. Proc. Phil. Acad., Ap. 1st, 1856. Type P. effusa Lea, sp. unica. [FIG. 28.] [Illustration: FIG. 28.] _Shell_; small, swollen, with three whorls, the last much the largest. Above, rounded planulate, suture deep but not channelled. Below, not umbilicated, rounded convex. Aperture large, effuse, internally shining. Columella flattened, not folded; externally, in fresh specimens, greenish horn color; dead ones nearly white or waxy brown. Alt. 0.14 in., major diam. 0.2, min. diam. 0.16 in. _Soft parts._ Foot rounded truncate; short, bluish-ash color, darker on the margin. Lips broad, semilunar. Tentacles long, eyebearing; tips pale yellow, eyes black; a second pair sessile on the inner bases of the tentacles. Body delicate brownish olive green; a light streak just behind and outside of base of each tentacle. Localities: Sacramento River (Trask), Lea; Eagle Lake, Horn; Klamath Valley and Frazer Spring, Gabb. I am indebted to Mr. Wm. M. Gabb, of the State Geological Survey, for notes and drawings, confirmatory of previous doubts in regard to this rare and interesting mollusk. They were taken from the living animal, and coming from a distinguished Palæontologist may be relied on as correct. [N.B.—Through inadvertence in drawing, the shell is represented as reversed in the figure.]