THE NAUTILUS. 33^ 



I. in the Ten ]\[ile River, and are very abundant in the Bhick- 

 stone. 



Genus PiskUmn, Pfeiffer, 1821. 



This genus was confounded hy earlier writers with Tellina (a ma- 

 rine genus) and still later with Spliaerium. Pfeiffer first observed the 

 difference in both animal and shell and proposed the name of 

 Pisidium for this group. 



The animal of Sphaerium ha.- 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. e., 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 nuid 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 ahditum, 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, tW, height, w, breadth, too, 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, ^Massachu setts 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. I., although 

 not yet observed. P. abditum was described by Haldeman in Proc. 

 Acad. Nat. Sci. Phila. i, 53, 1841, and has twenty-five synonymous 

 names. 



{To be continued.) 



