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 uppér 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. 
0 BDO oS SOO eas FE 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 virginicum. Some speci- 
mens are higher than others and in this form approach com- 
pressum. 
3- Pisidium cruciatum Sterki, pl. xxxi, figs. 20, 21. 
Pisidium cructatum STERKI, The Nautilus, Vol. VIII, p. 97, pl. ii, figs. 
1-6, 13, 18a, 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 arather 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 
. Ks writer has not been able to satisfactorily trace the distribution of this species in 
e South. 
