FAMILY LYMN^EID^E 



93 



This species is the analogue of P. exacuous Say on the Pacific 

 Coast. The typical form from the Sacramento River and the vicinity 

 of San Francisco Bay is quite lenticular, with the periphery marked 

 by a (frequently marginated) keel. The shell itself is pale yellow or 

 white under a rather strong periostracum, which is 

 almost invariably more or less discolored by deposits 

 of a brown or black color. The sculpture is like that 

 of exacuous, the spiral sculpture being faint and FIG. 72. Plan- 

 sometimes absent in southern specimens, and tending orbisopercularis 



to be emphasized in northern ones. As a rule the , ' * 



Cooper, 

 margin of the aperture is not thickened except in 



young specimens which have been overtaken by drought or winter 

 before maturity. The keel is generally, but not always, present in 

 southern shells, but those from Oregon and northward show a tend- 

 ency to form a shell either without a noticeable keel, or with the keel 

 forming a margin to a plane upper surface, rather than a median 

 carina. When compared with Cooper's types in the National Museum 

 Mr. Vanatta's P. callioglyptus is seen to be identical. The variety ore- 

 gonensis retains the typical form but has stronger spiral sculpture. I 

 regard P. centervillensis of Tryon as a P. -planulatus with the keel 

 obsolete. What appear to be intergradational forms are numerous in 

 the large series of the National Museum ; though it would seem incred- 

 ible to any one possessing only the extremes that they can belong to 

 the same species. 



Planorbis (Gyraulus) hirsutus Gould. 



? Planorbis albus MULLER, Verm. Terr, et Fluv., n, p. 164, 1774. 



Planorbis hirsutus GOULD, Am. Journ. Sci., xxxvni, p. 196, 1840; Inv. 



Mass., p. 206, pi. xi, fig. 135, 1841. 

 Planorbis borealis (LOVEN) WESTERLUND, Mai. Bl., xxii, p. 77, 1875. 



Range. Washington, D. C. ! northward, east of 

 the Mississippi. Lake Superior ! Lake of the Woods ! 

 Lake Winnipeg and the Saskatchewan River ! Great 



Slave Lake ! 

 FIG. 73. Plan- 

 orbis hirsutus Var. borealis Westerlund : Port Clarence, Alaska. 



Gould. \. Northern Sweden. 



This species appears to be common only in New 

 England, if one may trust reports, and it is remarkable how few 

 records there are of it in the literature of American fresh water shells. 

 The shell is variable in form ; from having, in what I have re- 

 garded as the type, well rounded nearly cylindrical whorls, it varies to 

 a form more or less depressed and carinate and with an oblique aper- 



