20S INVERTEBRATA OF MASSACHUSETTS. 



Shell small, distorted, compressed, of a light greenish-3'ello\v 

 color, something like dirty, bleached wax ; right side in general 

 convex, but with the centre slightly indented, suture distinct ; 

 left or under side concave, forming an expanded umbilicus, ex- 

 hibiting about one half of each volution ; whorls four or five, 

 very much compressed, and reduced to a somewhat carinated 

 perimeter ; the last fourth of the outer whorl turns, somewhat 

 suddenly and quite remarkably, to the left, or downwards ; aper- 

 ture 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 slightly everted beneath ; surface finely wrinkled 

 by the lines of growth. Greater diameter j\ inch, small di- 

 ameter yj inch. 



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

 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 comes in contact with only 

 about half of its lower face. When immature it may be recognised 

 by its light color and concave form. Scattered hau's may often be 

 observed upon its surface. It has a general resemblance in its struc- 

 ture to P. exacidas, but the constantly sharp edge of that species is a 

 never failing mark of distinction. I must at present regard the P. 

 virens of Adams (Bost. Journ. Nat. HisL., iii. pi. 3, f. 16) as a variety 

 of this species, in which tlie last whorl is not remarkably diverted 

 from its regular course. 



Planorbis exacutus. 



Shell lenticular, umbilicated ; xohorls Jour, broader than high, 

 gradually thinning to a sharp edge. 



Figure 137. 



State Coll., No. 95. Soc. Cab., No, 1267. 



Planorbis exacuous, Say ; Journ. Acad. JVat. Sc, ii. 1G6. 



