210 INVERTEBRATA OF MASSACHUSETTS. 



State Coll., No. 77. Soc. Cab., No. 1283. 



Planorbis parvus, SAY ; Nicholson's Encyc., (Amer. ed.), iv. pi. 1, f. 5. 



Shell very small and compressed, discoidal, light-yellowish 

 horn-color ; right side nearly plane, but excavated at the centre ; 

 left side broadly concave ; whorls four, almost equally exhibited 

 on both sides, the outer one usually somewhat angulated at its cir- 

 cumference ; surface minutely marked by the lines of growth, 

 shining, clear ; aperture rounded, rather longer than broad, not 

 inclining to either side, its plane very oblique ; lip sharp, slightly 

 reflected on the left side ; within bluish-white. Greatest diameter 

 J inch, lesser diameter T l j inch, but generally much smaller. 



Animal whitish, dusky above, with a still darker line at tip of 

 tentacula. 



Abundant in brooks and ponds. 



This is the smallest shell of the genus which we have, unless, per- 

 haps, it be P. exacutus, which is commonly found of as small a size. 

 It is not difficult to be recognised by its regular figure, and its very thin, 

 compressed appearance. P. deflectus, armigerus, exacutus, and hirsii- 

 tus, all have marked peculiarities, which at once separate them from 

 this undistinguished species. 



PLANORBIS DILATATUS. 



Shell small, circumference carinated, flat above, convex below, 

 and with a small, deep umbilicus ; whorls three ; aperture large, 

 expanded. 



FIGURE 140. 



State Coll., No. 75. Soc. Cab., No. 2399. 



Shell small, of a yellowish green-color, minutely wrinkled by 

 the lines of growth ; spire flat, composed of not more than three 

 whorls, separated by a well-defined suture ; the outer whorl has 

 a sharp margin on a level with the spire, diminishing near, but 

 still modifying, the aperture ; below this line the whorl is very 

 convexly rounded so as to encircle a small, deep, abruptly formed 

 umbilicus. This whorl rapidly enlarges, and terminates in a very 

 large, not very oblique aperture, with the lip expanded so as to 



