PERISTOMATA. MOLLUSCA. AMNICOLA. 229 



Animal having an elongated foot, rounded posteriorly, with 

 each anterior angle produced laterally ; head half the breadth of 

 the foot, and protruding beyond it ; tentacula short, filiform, un- 

 equal ? the eyes seated at the side of the external base ; ovipa- 

 rous. Inhabits fresh water. 



That this group of small shells should be separated from PALUDINA 

 and also from CYCLOSTOMA, in which genus they were included by 

 Cuvier, is clear from the structure of the operculum, but more espe- 

 cially from the structure and habits of the animal. Among the differ- 

 ences the following are the most obvious : In this genus, the head pre- 

 cedes 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 difference. The animal has the 

 power of rising and swimming in an inverted posture at the surface of 

 the water, which the true Paludina never does. So far as observation 

 has yet gone, the Amnicola 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 

 concurs with me in instituting the genus AMNI'COLA. Its position 

 seems to be intermediate between PALUDI NA and MELA'NIA. 



Under this genus will come P. porata, lustrica, grana, and limosa 

 of Say ; Nicklinidna of Lea ; and Cincinnatiensis of Anthony, and 

 perhaps some of the sub-globular MELANIN. 



The genus NEMATURA of Benson includes shells very similar to 

 these, but they are said to have the last whorl contracted, as it ap- 

 proaches the aperture, 



AMNI'COLA PORA'TA. 



Shell small, sub-globose, thin, smooth ; whorls four, very con- 

 vex, suture deep ; aperture nearly circular ; inner lip barely 

 touching the preceding whorl; umbilicus large. 



FIGURE 157. 

 State Coll., No. 64. Soc. Cab., No. 1361. 



Paludina porata, SAY ; Journ. Jlcad. Nat. Sc., ii. 174. 



