Fresh- Water Shells. . 213 



apex slightly elevated, bluntly rounded a little behiud, and to the 

 right of the centre ; stages of growth visible ; epidermis coarse and 

 strong, rough, dusky yellowish-brown, extending beyond the 

 margin of the testaceous matter, and insensibly coalescing with it 

 on all sides, which are inclined to turn upwards ; within glistening, 

 polished. Length j\ inch, height J^ inch, breadth -f^-^ inch. 



Found in a rivulet in Andover by Mr. K. Prescott, of the 

 Theological Seminary ; and also found by Professor Adams in 

 Mansfield ; and by myself, in Fresh Pond. 



It differs from all other described species in its depressed form 

 its obtuse apex, and its coarse epidermis projecting beyond the 

 margin ; and, as this extends in the direction of the plane of the 

 object to which it is found attached, and not in continuation of 

 the convex form of the shell, the edges seem to be turned upwards. 

 A. rivtdaris, Say, is narrower, and has the sides nearly parallel. 

 A. tardus, Say, has its apex prominent, acute, and farther behind 

 the middle. 



Genus VALVx\TA. Muller. 



Shell conical, whorls cylindrical, loosely coheriny ; aperture 

 circular, its maryin entire ; operculum orbicular. 



Valva'ta Tricarixata, — (Say.) 



Shell small, depressed, thin, transparent and shinino-, 



of an emerald or light pea-green color ; whorls three or 

 four, flattened at the summit, faintly marked by lines of 

 growth, and separated by a distinct suture ; each of the 

 interior Avhorls has one or two prominently raised, round- 

 ed, revolving lines or keels, and the exterior one has three 

 one of which issues from the lower junction of the lip and borders 

 the umbilicus ; a second originates from the upper junction of the 

 lip, and circumscribes the whorl ; the third midAvay between this 

 and the suture, thus giving the whorls a prismatic or quadrangular 

 instead of a cylindrical appearance ; aperture circular, modified by 

 the keels ; lip simple, surrounding the aperture, except a small 

 space between the two lower keels ; umbilicus broad, deep, tunnel- 

 shaped. Height j\ inch, breadth £-^ inch. 



It is found in most of our small lakes, usually under stones, or 

 sheltered by the deserted shells of some of the fresh-water 

 mussels. 



The shell is usually rendered somewhat opaque by an earthy 

 coating, which seems to answer Ihe purpose of an epidermis; but, 



