Fresh- JVater Sheik. 207 



somewliat carinatecl perimeter; the lastfourtli of the outer whorl 

 turns, somewliat suddenly and quite remarkably, to the left, or 

 downwards ; aperture large, ovate, lip commencing below the 

 carina, and embracing but a very small portion of the preceding- 

 whorl ; much narrower from side to side, its plane very oblique 

 to the axis of the shell ; lip simple, very shghtly everted beneath ; 

 surface finely Avrinkled by the lines of growth. Greater diameter 

 j\ inch, small diameter j\ inch. 



Animal dusky above, and with a still darker line to tip o^ 

 tentacula. 



Found in all our ponds, clinging to sticks, stones, &c. 



It is distinguished at once, except in its very early stages, by 

 the remarkable manner in which a portion of the last whorl is 

 diverted from its regular course, downwards, if we consider the 

 shell to be lying on its concave face. It is almost entirely turned 

 off from the preceding whorls, so that the aperture cornea in 

 contact with only about half of its lower face. "When immature 

 it may be recognised by its light color and concave form. Scat- 

 tered hairs may often be observed upon its surface. It has a 

 general resemblance in its structure to P. exacutus, but the cons- 

 tantly sharp edge of that species is a never failing mark of dis- 

 tinction. I must at present regard the P. virens of Adams {Bost. 

 Journ. Nat. Hist., iii. pi. 3, f. 16.) as a variety of this species, in 

 which the last whorl is ]iot remarkably diverted from its rep-ular 

 course. 



Plakorbis Exacutus. — (Say.) 



/§^ I) '^ Shell lenticular, light transparent horn- 



^^ v| w color ; whorls fouj-, flattened so that the 

 width of each is at least twice its depth, the upper and lower 

 surfaces convex, and brought to a sharp exterior edge ; the last 

 half of the outer whorl deflected, so that the termination of the 

 sharp edge is on a level with the lower surface of the jDreceding 

 whorl ; inner whorls slightly depressed, and somewhat more 

 rounded ; suture moderately impressed ; striss of growth faint ; 

 beneath abruptly umbilicated, displaying the edges of all the 

 whorls within ; aperture very oblique and angular ; edge very 

 sharp, below running forwards a little along the umbihcal edge 

 of the preceding whorl, then crossing obliquely forwards and 

 upwards, leaving a callus, it passes off^ again a little below its 

 carinated edge. Longest diameter \ inch, shorter diameter -^-^ 

 inch. 



