r: 



PLANORBIS. 49o 



Planorhts vircna, Adams, Am. Journ. Sc. [i], xxxix. 274 (1840) ; Bost. Journ. iii. 326, 



pi. 3, fig. lo (1840). -De Kay, N. Y. Moll. GO (1843). 

 Planorhls ohiiqnns, De Kay, N. Y. jMoH. 62, pi. 4, figs. 57 n, b (1843). 

 Naulilina defltcta, Chexu, ^lan. dc Concli. ii. 482, fig. 3.')66. 



Shell small, distorted, compressed, of a light greenish-yellow 

 color, something like dirty, bleached wax ; right side in ^..^ .^^ 

 general convex, bnt with the centre slightly indented, sut- ^^^ 

 nre distinct; left or under side concave, forming an ex- F.dejiec- 

 ])anded umbilicus, exhibiting about one half of each volu- 

 tion ; whorls four or five, very much compressed, and reduced to a 

 somewhat carinatcd perimeter ; the last fourth of the outer whorl 

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

 downwards ; aperture large, ovate, lip commencing below the cari- 

 na, 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, three 

 tenths of an inch ; smaller diameter, one fifteenth of an inch. 



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

 tacula. 



Found in all our ponds, clinging to sticks, stones, <fec. 



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 recognized 

 by its light color and concave form. Scattered hairs may often be 

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

 ture to P. exacutus, 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. Hist." iii. pi. 3, fig. 16) 

 as a variety of this species, in which the last whorl is not remark- 

 ably diverted from its regular course. 



[This species is said to range from Great Slave Lake to the Dis- 

 trict of Columbia, and from New England to Nebraska. 



Planorbis exacutus. 



Fig. 137. 



Shell lenticular, umbilicated ; whorls four, broader than high, gradually thinning 

 to a sharp edge. 



