THE NAUTILUS. 33 
I. in the Ten Mile River, and are very abundant in the Black- 
stone. 
Genus Pisidium, Pfeiffer, 1821. 
This genus was confounded by earlier writers with Tellina (a ma- 
rine genus) and still later with Sphaerium. Pfeiffer first observed the 
difference in both animal and shell and proposed the name of 
Pisidium for this group. 
The animal of Sphaerium has the lobes of the mantle united pos- 
teriorly, into a tube, single at the base, but separated into two 
siphons at the extremities, while in Pisidium it is united its entire 
length. 
The shells of Sphaerium have the beaks central, dividing the 
hinge margin into equal parts, and the cardinal teeth are situated 
immediately under the beaks; in Pisidium the beaks are terminal, 
i. €., nearer the posterior extremity; the cardinal teeth also are 
terminal and the ligament is on the shorter side. The teeth of 
Pisidium are stronger and more robust in proportion to the size of 
the shell than in Sphaerium. The habits of the animals are the 
same, burrowing in mud or attached to the roots and stems of aquatic 
plants. The best time to collect these shells is from April to July. 
There are eight species in New England, three of which have 
been found in R. I. 
174.—Pisidium abditum, Haldeman. 
Shell rounded-oval, elongated, margins well rounded; beaks 
small, raised a little above the curve of the shell; surface smooth, 
not distinctly striated; epidermis generally straw color, but some- 
times dark and the surface rough and coarsely striated; cardinal 
teeth small, separate ; the anterior one larger and prominent ; lateral 
teeth short. Length, yo%, height, 5%, breadth, ric, inch. Inhabits 
nearly all of North America, is very common and is found in 
swamps and on the margins of small streams. 
P. aequilaterale, compressum, ferrugineum, and ventricosum are- 
species which occur in Maine, Massachusetts and New York, but 
have never been found in R. I. They are widely distributed over 
other parts of the country and may possibly inhabit R. L., although 
not yet observed. P. abditum was described by Haldeman in Proc. 
Acad. Nat. Sci. Phila. i, 55, 1841, and has twenty-five synonymous 
names. 
(To be continued.) 
