98 THE NAUTILUS. 
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 isola- 
ted; muscle insertions visible; ligament, short, strong. 
Size; long 1°9, alt. 1:9, diam. 1-4 mill. 
Soft parts slightly yellowish. 
Habitat: Tuscarawas River, at New Philadelphia, Ohio. 
Figure 1 represents an adult specimen, 2 the posterior, 3 the dor- 
sal aspect, 4-6 a young example, 0°8 mill. long; fig. 13 the hinge ; 
13a the dorsal aspect of the right cardinal tooth with an indenta- 
tion ; figs. 1-6: scale 15 x1. 
The singular shape of the umbones is so characteristic that this 
species will be recognized at once, and cannot be mistaken for any 
other. And also in the formation of the hinge it is quite unlike 
any other Pisidum, 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 cardinal teeth ; yet this 
is not the place to discuss the subject at length. 
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°6 to. 
2-1 mill. (One specimen 2°1:2'1:1°6, another 1°7:1°7:1°3 mill.), 1°9 
being about the average; the margins may be rather obtuse or 
somewhat acute, the beaks more or less prominent, and the anterior 
nodule more or less marked. Asa rule there are no outer lateral 
teeth in the right valve, yet traces of them may be seen in some 
specimens. Also in the formation of the cardinal teeth there is 
some variation, as the one in the right valve may be indented in its 
middle, at the angle, and so there are apparently two teeth (fig. 
13a). 
Pis. cruciatum is not rare in the Tuscarawas River, where, so far, 
it has only been found, but probably it has a wider distribution, and 
may even have been collected and taken for the young of some other 
species, owing to its minute size. In October and November, 1891, 
it was first found, and some specimens were sent to several conchol- 
ogists, so to Mr. E. W. Roper, who also recognized it as a new spe- 
cies. In 1893 and 1894 about a hundred were collected, in all 
stages of growth. Only few of them are quite mature, and almost 
globular, most adolescent, and, though apparently old, much less 
inflated. They live in mud among aquatic plants and dead leaves, 
