PERISTOMATA. MOLLUSCA. VALVATA. 225 



rounded a little behind, and to the right of the centre ; stages of 

 growth visible ; epidermis coarse and strong, rough, dusky yel- 

 lowish-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 

 T \ inch, height , l ff inch, breadth T a g* 7 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 v/hich 

 it is found attached, and not in continuation of the conxex form of the 

 shell, the edges seem to be turned upwards. A. rivularis, Say, is 

 narrower, and has the sides nearly parallel. A. tardus, Say, has its 

 apex prominent, acute, and farther behind the middle. 



FAMILY PERISTdMATA, LAM. 



Shell conical or sub-discoidal, the margins of the aperture united poste- 

 riorly ; opercidated ; inhabiting fresh water. 



\ 

 GENUS VALVATA. MULLER. 



Shell conical, whorls cylindrical, loosely cohering ; aperture 

 circular, its margin entire ; operculum orbicular. 



VALVATA TRICARINATA. 



Shell sub-discoidal, thin, pale pea-green; whorls three, the last 

 tri-carinate ; umbilicus large. 



FIGURE 156. 

 State Coll., No. 67. Soc. Cab., No. 1542. 



ilostoma tricarinata SAY; Nicholson's Encyc., (Amer. ed.,) Art. Conchology, 

 Journ. Aead. Nat. Sc., i. 13. 



Ivata tricarinata, SAY ; Journ. .lead. Nat. Sc. DESHAYES ; LAM. ; An. sans 

 l r ert., (2d edit.), viii. 507. 

 Ivata carinata, SOWERBY ; Genera, f. 2. 



Shell small, depressed, thin, transparent and shining, of an 

 ;rald or light pea-green color ; whorls three or four, flattened 

 29 



