TAMILY LIMNIAD^ PLANORBIS. 63 



• Planorbis EXACUTUS. 



PLATE IV. FIG. 62. i. B. 



(STATE COLLECTION.) 



Phnorbis exacutxis. Sat, Jour. Acad. Nat. Sci, Vol. 2, p. 164. 

 P. id. Adams, Am. Jour, of Sci. Vol. 40, p. 269. 



P. id. GoDLD, Invertebrata of Mass. p. 208, fig. 137. 



Description. Shell thin and fragile, much depressed, lenticular. Whorls four; upper and 

 under sides slightly conve.Y, 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 ; tiie upper termination just 

 including the acute edge of the penultimate whorl. 



Color, light corneous. 



Diameter, 0-2 -0-3. Height, 0-07 -O'l. 



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. 



(STA.TE COLLECTION.) 



Planorbis panms. Say, Nich. Ency. Ed. Am. Vol. 4, pi. 1, fig. 5, 

 P. id. Gould, Invertebrata of Mass. p. 209, fig. 139. 



Description. Shell small, tliin, 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 reddieh 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. It is one of the smallest 

 of the group. 



