FAMILY LIMNIADZ — PLANOBBIS. 63 
PLaNoRBIS EXACUTUS. 
PLATE IV. FIG. 62. a. B. 
(STATE COLLECTION.) 
Planorbis exacutus. Say, Jour. Acad. Nat. Sci. Vol. 2, p. 164. 
P. id. Apams, Am. Jour. of Sci. Vol. 40, p.'269. 
P. id, Goutp, Invertebrata of Mass. p. 208, fig. 137. 
Description. Shell thin and fragile, much depressed, lenticular. Whorls four; upper and 
under sides slightly convex, flattened to the periphery, and forming an acute edge which is 
continued on the middle of the aperture, which is below the plane of the transverse diameter : 
surfaces of the whorls transversely striated. Umbilicus regular, showing all the volutions to 
the apex. Suture moderately impressed. Whorls wider than high. Aperture subtriangular, 
oblique. Lip angulated in the middle, arched near its lower tip ; the upper termination just 
including the acute edge of the penultimate whorl. 
Color, light corneous. 
Diameter, 0:2-0°3. Height, 0°07 -0°1. 
Common in ponds and ditches. I have obtained it from the northern and western districts. 
It is one of the most fragile and most depressed of all the species. 
PLANORBIS PARVUS. 
PLATE IV. FIG. 58. 
(STATE COLLECTION.) 
Planorbis parvus. Say, Nich. Ency. Ed, Am. Vol. 4, pl. 1, fig. 5. 
Yeh id. Goutp, Invertebrata of Mass. p. 209, fig. 139. 
. Description. Shell small, thin, depressed, discoidal; upper side nearly plane, but concave 
in the centre. Umbilicus broadly concave, and both sides exhibiting all the volutions. Whorls 
four ; the body-whorl obtusely carinated on its circumference, and with impressed incremental 
lines. Aperture rounded, oblique ; its upper and lower margins in the plane of the transverse 
diameter of the shell. Lip sharp, not thickened. 
Color. Varying from reddish brown to yellowish or olivaceous. Animal whitish, darker 
above. 
Diameter, 0°08 -0°1. Height, 0°02 - 0°04. 
Common. I have received specimens from the Mohawk and Connecticut rivers, which 
only differ from the above in having the upper edge of the mouth nearly in the centre of the 
last whorl. I refer them with doubt to the above named species. Itis one of the smallest 
of the group. 
