126 THE NAUTILUS. 



PISIDITJM STRENGII, N. SP. 



BY DR. V. STERKI. 



Mussel of moderate size, regularly inflated, rather short ; beaks 

 slightly posterior, small, narrow, approximate, somewhat projecting 

 over the hinge margin ; superior and inferior margins well curved, 

 the supero-anterior slightly so and forming a steep slope to the some- 

 what angled anterior end ; the posterior end subtruncate ; scutum 

 and scutellum indistinct ; angles in front of and behind the beaks 

 slight, rounded ; shell rather thin, translucent ; surface very finely 

 striate, appearing smooth, with a few fine, irregular lines of growth, 

 and with a slight, dull gloss ; color of epiconch pale horn shading 

 into grayish, whitish or yellowish ; nacre almost glassy, muscle scars 

 very slight ; hinge fine, plate narrow; cardinal teeth small, thin; 

 the right one curved, its posterior end deeply cleft, the left anterior 

 curved or almost straight, the posterior short, oblique ; lateral teeth 

 small, somewhat pointed, the outer ones of the right valve quite small 

 but distinct ; ligament small. Long. 4, alt. 3.7, diam. 2.6 mill, 

 (average). 



Hab.: Michigan to New York, Ohio and Indiana. It seems to be 

 a form mainly inhabiting smaller lakes. Perch lake, Mich., col- 

 lected by Mr. L. H. Streng (in whose honor the species is named) 

 and Dr. Kirkland ; Reed lake and Little Bostwick lake, Mich.; 

 Bass lake, Ind. (among shells taken from the stomach of catfish), by 

 Mr. L. E. Daniels; Meyer's lake, Ohio (Sterki); Little Lakes, 

 N. Y., found among materials in the collection of the late Dr. Jas. 

 Lewis, now in possession of Mr. Bryant Walker. 



Pisidium strengii has a resemblance to some smaller forms of 

 P. abditnm Hald., nov-eboracense Pr., and with politum Sterki, but 

 can be recognized by its high form, the small, approximate beaks, 

 especially noticeable in front (or rear) aspect, the peculiar, dull gloss 

 of the surface, and the strongly cleft posterior end of the right car- 

 dinal tooth. The latter feature was found constant in normal speci- 

 mens from widely distant places ; but it may be added that the 

 species appears to be particularly inclined to abnormal, and even 

 monstrous formation of the cardinal teeth. As to size, shape and 

 general appearance, the mussel does not vary greatly, so far as seen. 



This species was first noticed as different from others, and named, 

 in 1895. Yet, considering the great variability of our Pisidia as 

 well as the fact that it does not show a striking variance from others, 

 in general appearance, it was thought best to wait until sufficient 

 evidence was gained of its being really distinct. 



