THE NATURAL HISTORY SURVEY. 399 



ending in a nodule of which the posterior is larger and more 

 prominent; nave rather thick, whitish; hinge very stout, list 

 strong; cardinal teeth rather large, and strongly projecting in- 

 ward from the hinge list; in the right valve one, strongly 

 curved, posterior part thick, anterior thin, lamellar, the ends 

 united by a lamella so as to form a deep groove into which the 

 posterior tooth of the left valve articulates; in the left valve 

 two, the posterior (inferior) stout, massive, the other, superior 

 and a little anterior, rather short, fine, lamellar, oblique, little 

 curved; lateral teeth large, high and pointed in the left, 

 strongly projecting inward in the right valve, the latter with 

 no (or only traces of) outer teeth, and a deep groove; between 

 the cardinal and the lateral teeth the hinge-list is rather deeply 

 excavated, so that all teeth are markedly isolated; muscle in- 

 sertions visible; ligament short, strong." (Sterki.) 



Length, 1.90; height, 1.90; breadth, 1.40 mill. 

 " 2.10; " 2.10; " 1.60 " 

 " 1.70; " 1.70; " 1.30 " 



Animal: "Soft parts slightly yellowish." (Sterki.) 



Distribution: Tuscarawas River, New Philadelphia, Ohio 

 (Sterki); Joliet, 111. (Ferriss); Lilycash Creek (Handwerk). 



N Geological distribution: Pleistocene; Lower Loess of Illi- 

 nois. (Sterki.) 



Habitat: "Living in mud among aquatic plants and dead 

 leaves, and as a rule covered with a black or brown coat, some- 

 times so thick that they appear to be globules of dirt." (Sterki.) 



Remarks: "The singular shape of the umbones is so char- 

 acteristic that this species will be recognized at once, and can- 

 not be mistaken for any other. And also in the formation of 

 the hinge it is quite unlike any other Pisidium, so that it holds 

 a peculiar position in the genus. In the hinge structure it is 

 very illustrative and instructive for the understanding of the 

 different forms of teeth." .... 



"Our species is rather variable, even so that all specimens 

 from one place in the river are different in size and shape 

 from those of another place scarcely half a mile distant. It 

 measures from 1.60 to 2.10 mill., 1.90 being about the aver- 

 age; the margins may be rather obtuse or somewhat acute, 

 the beaks more or less prominent, and the anterior nodule 

 more or less marked. As a rule there are no outer lateral teeth 

 in the right valve, yet traces of them may be seen in some spec- 



