FAMILY LYMN^EID^E 9 1 



diminish in size much more rapidly than in the variety. The campan- 

 ulate aperture is about the same size in both forms, but seems larger 

 in the type because the rest of the shell is so much more tightly 

 wound. The suture on the apical side seems deeper and wider than 

 in the type. Nine specimens of the variety were obtained, and I sug- 

 gest for it the name rudentis, from the similarity of the whorls to a 

 coiled hawser. 



Planorbis (Menetus) exacuous Say. 



Planorbis exactions Say, Journ. Acad. Nat. Sci. Phila., II, p. 168, Jan., 1821 

 (Lake Champlain) ; Long's Exp. Rep., II, p. 261, 1824. 



Planorbis exacutus Gould, Inv. Mass., p. 208, fig. 137, 1841. — Haldeman, 

 Mon. Limn., p. 21, pi. iv, figs. 1-3, 1844. 



Pahtdina hyalina Lea, 1839 (scalariform monstrosity). 



Range. — Northern United States, east of the Rockies; Canada, 

 etc., south to New Mexico. 



Lake Superior to Lake Winnipeg ! Manitoba generally ; Moose 

 Factory, Hudson Bay ! Left bank of the Yukon below Fort Yukon, 

 Alaska, in Pleistocene marl (A. J. Collier) ! 



Variety megas Dall, nov. : Birtle, Manitoba (R. M. Christy). 



This species has a number of varieties both in size and form. The 

 typical shell is of a pale brownish horn color, with a somewhat glisten- 

 ing surface, rather rudely striated by the incremental lines, and with 

 faint, almost microscopic, revolving striae. The form is lenticular, 

 coming to an acute angle at the periphery. In 1863 I found in the 

 vicinity of Marquette, Michigan, an unusually depressed brownish 

 variety in which the peripheral keel was delicately serrate. In the 

 northwestern part of its range the tendency is for the species to become 

 whitish and of a larger size than the average New York or New 

 England specimens. This reaches its maximum in specimens col- 

 lected in Manitoba by Mr. R. Miller Christy, for which I propose the 

 varietal name megas. The comparative measurements are as follows : 



The variety is of a slightly milky translucency ; on the base the 

 whorl is more or less impressed within the peripheral keel and the 

 spiral striation is much more marked than in the typical form. 



Binney has united with this species Planorbis lens Lea, 1839 (not 

 Brongniart, 1S10) = P. lenticularis Lea, 1844 (not Schlotheim, 

 1818) = P. brongniartiana Lea, 1842; but an examination of Lea's 



