GASTEROPODA. 113 
It is about the size of P. deflectus, Say, but is less depressed, the 
whorls more cylindrical, not carinated at periphery. 
Figures 131, 131 a, 131 0, three views of the shell. 
PLANORBIS OPERCULARIS (Gould). 
Testa parva, valde depressa, castanea, supra planulata ; apice depresso : 
subtus convexa, late umbilicata, ad peripheriam marginata et obtuse 
carinata: spira anfractibus quatuor, suturd bene impressa sejunctis : 
apertura transversa, subrhiomhondea. 
Plancrbis opercularis, Goutp ; Proceed. Boston Soc. Nat. Hist. ii. 
212. June 1347. Expedition Shells, 42. 
SHELL small, dextral, much depressed, lenticular, with a prominent, 
blunted keel at the periphery, defined by a marginal compressed line ; 
tip sunken; beneath umbilicated for about one-third the breadth of 
base, showing three volutions, convex, surface rather rude and in- 
dented, marked with irregular, coarse, much arcuated lines of growth, 
and here and there a few obscure, raised, revolving lines; colour dark 
chestnut brown, a little clouded; whorls above four, slightly convex, 
suture well defined, impressed: aperture transversely subrhomblic, lip 
above slightly declining, at periphery acute angled, beneath arched, 
lips embracing three-fourths of that part of the whorl which is 
beneath the carina. 
Length one-fourth of an inch; diameter one-sixteenth of an inch. 
Obtained from the Sacramento River, California. 
Allied to P. exacutus, \yut is larger, less compressed and less delicate, 
and the periphery instead of being sharp-edged, has a blunted keel like 
P. carinatus. 
Figures 132, 132 a, 1326, three views of the shell. 
PLANORBIS CORPULENTUS (Say), Appendix to Long’s Exped., 362 
29 
