FAUNA CANADENSIS. 503 



more rounded and ventricose form, by having the spire on one side 

 perfectly flat and exhibiting all the whorls ; whilst the other side is 

 depressed but widely open. The flat side of the shell is carinated, and 

 the aperture angulated opposite to the carina. The whorls are crossed 

 by numerous fine curved lines. Colour pale brown or olive. 

 Hah. — Rare in Toronto Bay in one to two fathoms. 



20. Planorlis parvus, Say. 



This little species is easily made out by its small, thin, discoidal 

 shell, both sides of which exhibit all the volutions. The upper side 

 is nearly flat, though concave centrally, and the umbilicus is shallow 

 and wide. The aperture is oblique and rounded, and the lip is acute 

 and thin. Colour yellowish or pale brown. The larger specimens 

 attain a diameter of nearly two lines, the smaller about one line. 



Hah. — Not uncommon in Toronto Bay in from one to three 



fathoms. 



21. Valvata tricarinata, Say. 



Shell small, thin, slightly elevated, with a large and deep umbilicus 

 exposing the whorls. Whorls three, strongly keeled with two 

 cariase, and marked with transverse curved lines. Apertvire round, 

 complete ; lip thin. Suture deeply impressed. Colour pale greenish- 

 brown or brownish white. Full-grown specimens about one-tenth of 

 an inch in height by nearly two lines in diameter. 



Hob. — Abundant, crawling over aquatic plants, in from one to 

 eight fathoms. Most abundant in Toronto Bay. 



22. VaUafa sineera, Say. 



Shell small, thin, slightly elevated, with a large and deep umbilicus 

 exposing all the volutions. Whorls three or nearly four, rounded 

 and not carinated, marked across with very numerous and regular 

 curved striae. Aperture round, complete, the lip thin and acute. 

 Colour light brownish, or brownish-green. Size variable, large speci- 

 mens being almost a line in height with a diameter of nearly two 

 lines. 



This species is very closely allied to Valvata tricarinata, Say, from 

 which it difiers almost solely in the fact that the whorls are rounded 

 instead of being keeled. This peculiarity gives a very distinct /acies 

 to the two shells, and there is not the smallest difficulty in picking 

 out well-marked examples of each. A number of intermediate 

 examples, however, occur, in which there are more or less faintly or 



