2 AMNICOLA. 



from 4 to T convex whirls, separated by a distinct suture ; aper- 

 ture oblique, peritreme simple, detached, or but slightly con- 

 nected with the body whirl, and usually by a very 

 Fig- 2. small portion of its circumference posteriorly ; base 



usually perforate ; operculum thin, corneous, com- 

 posed of a few spiral volutions. 

 ^Amnieoia'^ -4. decisa and lustrica, where I have observed 



them, live upon the inferior surface of stones in 

 running water. They are tolerably active, and retract sud- 

 denly when a shadow is passed over them. The ova are depo- 

 sited in the mouth of March, in small oblong detached glairy 

 masses, each of which contains apparently but one germ, which 

 is situated at the larger end of the mass. The color of the germ 

 is orange, of the mass yellowish transparent, with a dark central 

 line upon the surface from end to end. The progression, at least 

 of the short species, is performed upon the foot alone, with a 

 uniform gliding motion, as in Physa. (Haldeman.) 



That this group of small shells should be separated from Palu- 

 dina and also from Cyclostoma, in which genus they were included 

 by Cuvier, is clear from the structure of the operculum, but more 

 especially from the structure and habits of the animal. Among 

 the differences, the following are the most obvious : In this genus, 

 the head precedes the foot in progression ; in Paludina it is the 

 contrary ; in this the tentacula are all the way of a size, and 

 without any enlargement for the reception of the eyes, instead of 

 being tapering, with a niche for the eyes ; they are also frequently, 

 if not always, unequal in length ; perhaps this is a sexual differ- 

 ence. The animal has the power of rising and swimming in an 

 inverted posture at the surface of the water, which the true Palu- 

 dina never does. So far as observation has yet gone, the Amni- 

 cola is oviparous, while the true Paludina is ovo-viviparous. It 

 is found crawling upon stones, sticks, and aquatic plants, while 

 Paludina remains upon the mud, and is usually observed partly, 

 or entirely, imbedded in it. On these grounds Mr. Haldeman 



