THE NAUTILUS. 99 



less expanded aperture. The lip joins the body-whorl at a more 

 obtuse angle and lacks the basal expansion characteristic of the lake 

 form. The spire is also less produced. Compared with the typical 

 form, this variety is easily distinguished by its heavy, solid, opaque 

 shell. 



NEW PISIDIA.. 



BY DR. V. STEKKI. 



Pisidium tenuissimum, n. 



Rather small, little to strongly oblique, moderately to rather well 

 inflated, with the edges usually acutish, elongated or rather short, 

 rhomboid to oblong-ovoid in outline, hinge margin little, inferior mod- 

 erately curved, posterior end rounded or subtruncate obliquely in a 

 postero-anterior direction with a rounded angle above ; anterior more 

 or less curved, truncate obliquely with the rounded-angular end 

 inferiorly, or the whole anterior part rather regularly parabolic, with 

 the end in the longitudinal median line ; beaks slightly posterior, 

 somewhat broad, moderately elevated over the hinge line, somewhat 

 mam miliar ; surface very finely, almost regularly striated, highly 

 polished ; color horn to smoky, or to greenish, or to light grayish ; 

 shell very thin, translucent ; hinge very fine, plate very narrow, 

 cardinal teeth very small, short, thin, scarcely or slightly curved; 

 those of the left valve very close together, longitudinal-parallel, the 

 upper little posterior; lateral teeth rather long, markedly straight, 

 slender, thin, with short cusps ; also the outer ones in the right valve 

 quite distinct ; ligament fine. 



Long. 3.4, alt. 2.8, diam. 2.1 mill. 



Habitat : Straits and Orchard lakes, in Oakland Co., Michigan, 1 

 collected by Messrs. Walker and Sargent ; Pine Lake, Marquette 

 Co., Mich. (Upper Peninsula), and other waters in the same state ; 

 also seen fossil. 



As pointed out in the description, the species is very variable in 

 shape, and extreme forms, if found separate, might well be taken for 

 different species. The one from Pine Lake (collected by Mr. 

 Walker) is especially notable : the beaks are narrower and more 



1 Close together, but at the heads of different river systems. 



