398 THE CHICAGO ACADEMY OF SCIENCES. 



pyramidal teeth in the left valve; the right valve tooth is con- 

 stricted in the center of the arch and gradually enlarges to- 

 ward the distal end of the arch, the right arm of arch being 

 the longer and reaching nearly to the base of the hinge plate; 

 the upper left valve tooth is somewhat gourd-shaped, begin- 

 ning small at the upper margin of the hinge plate and gradu- 

 ally enlarging to about the center of the plate; the lower left 

 valve tooth is large, solid and pyramidal; lateral teeth elevated 

 above the valve edge, triangular; the entire hinge plate about 

 the lateral teeth is enlarged, thick and heavy; cavity of the 

 beaks deep and full; nacre bluish-white, shining. 



Length, 4.50; height, 4.50; breadth, 3.10 mill. 

 " 4.00; " 4.00; " 3.00 " 



Animal: Not observed. 



Distribution: New England west to Washington, Michi- 

 gan south to Virginia and Kansas.* 



Geological distribution : Pleistocene; Loess. 



Habitat: In soft mud in creeks, rivers and lakes. 



Remarks: Variabile does not appear to be common in 

 this region, although it has been found rather widely distrib- 

 uted, specimens having been collected in Lake Michigan, Lily- 

 cash Creek, Du Page River and Rock Run. It is therefore 

 found in the southern and western regions. It is quite a dis- 

 tinct shell, distinguished from compressum by its less trigonal 

 form, and being more oblique than virgmicum. Some speci- 

 mens are higher than others and in this form approach com- 

 pressum. 



3. Pisidium cruciatum Sterki, pi. xxxi, figs. 20, 21. 

 Pisidium cruciatum STERKI, The Nautilus, Vol. VIII, p. 97, pi. ii, figs. 

 1-6, 13, 13a, 1895. 



Shell: "Minute, inequipartite, oblique, subtriangular in 

 outline, high, ventricose, regularly and comparatively coarsely 

 striated, straw-colored; anterior part moderately long with 

 an oblique, nearly straight edge above and the end rounded; 

 posterior part short, the end somewhat obliquely truncate, 

 superior margin rather strongly curved, scutum scarcely, scu- 

 tellum little marked, the latter forming a rather distinct angle; 

 inferior part moderately curved; beaks prominent, each with 

 two ridges diverging at nearly right angles, together forming 

 a cross on the upper aspect of the shell, each of the ridges 



*The writer has not been able to satisfactorily trace the distribution of this species in 

 the South. 



