88 THE NAUTILUS. 



edge of the plate, its posterior end much thicker, deeply grooved or 

 bifid ; there is an impressed groove between it and the edge of the 

 plate; left anterior strongly curved to sharply angular and bent up- 

 ward, the posterior oblique, curved ; '' lateral teeth " moderately 

 stout, the outer ones of the right valve distinct, the anterior left 

 cusp pointed, abrupt, the surfaces of all cusps rugulose ; ligament 

 rather strong. 



Long. 3, alt. 2.6, diam. 1.8 m., types, and average. 



Soft parts not examined. 



Habitat: North America; widely distributed and common in 

 Ohio, Michigan and Illinois, and probably over the Nearctic prov- 

 ince, seen also from New York, Canada and Alabama ; living in all 

 kinds of waters. The specimens regarded as types are from a brook 

 at Krumroy, Summit Co., Ohio, 1900 (No. 3265, collection of 

 Cycladidoe). 



This Pisidium was regarded as probably distinct years ago ; since 

 then the material has been looked over and compared dozens of 

 times. It seems to be the central form of a group which is one of 

 the most difficult to deal with of all our Pisidia. There are a host 

 of forms ranging under it or near it, really or apparently, and if only 

 a few specimens from various places were on hand, they might well 

 be regarded as distinct. But among the thousands seen, there are 

 more or less plainly intermediate forms. Some are still under doubt 

 and scrutiny, and waiting for additional material in order to ascer- 

 tain their true positions. Some forms have a slight resemblance 

 to some small forms of the polymorphous " abditum Hahl.," others 

 to small forms of noveboracense Pr. It may also be said that the 

 more typical specimens resemble Pis. pusillum Gmel. more than any 

 other of our Pisidia does. 



Var. (?) corpulentum n. Larger, more inflated, somewhat more 

 elongate and oblique, beaks generally somewhat more posterior, 

 large, prominent ; concentric striae coarser, surface more dull, color 

 whitish to yellowish, to grayish, and even bluish with lighter zones 

 along the margins. 



Hab.: Grand River, Grand Rapids, Mich., collected repeatedly 

 by Dr. R. J. Kirkland in good numbers ; a similar Pisidium is from 

 Sillycash creek, near Joliet, 111., collected by Messrs. J. H. Ferriss 

 and J. H. Handwerk. It seems that these forms are connected with 

 Pis. neglectum, although the Grand River, Mich, form, is considerably 

 different and appears to be distinct. 



