GEOLOGY OF LA SALLE COUNTY. 



Ill 



SUBFAMILY POMATIOPSIX.E — GENUS POMATIOPSIS, 



TRXON. 



Pomatiopsis lapidaria. Sa}'.— Shell turrited, thin, 

 smooth, volutions six, suture impressed, aperture long-i- 

 tudinallv ovate — orbicular, operculated, rather more 

 than one-third the leng-th of the shell, color brown, 

 leng-th one-fifth of an inch. Abundant. 



This species is long-er than A. -porata. I follow 

 Binnev in placing- it in the genus Pomatiopsis. The 

 animals are amphibious, and ma}- be found near our 

 rivers in moist places. They possess the power of 

 crawling- on the surface of the water in a reversed 

 position, shell downward. 



FAMILY STREPOMATID.E, HALDE- 



This family presents in its numbers and diversi- 

 fied forms one of the most interesting* studies in the 

 whole rang-e of American Conchology. There are 

 nearlv five hundred recog-nized species of Strepoma- 

 tida from Xorth America: the larg-er part from South- 

 ern waters, onlv a few being- found in the Ohio river 

 and its northern tributaries, and these are small com- 

 pared with those from Tennessee and Alabama. In 

 the East, the St. Lawrence river and its branches 

 form the northern limit of the famil}-; and in the 

 West, the northern boundary of the United States, 

 beyond which, I believe, none have been found. The 

 Ohio river seems to form a dividing- line also, both 

 northern and southern species fading- out as they 

 approach this stream, Thev are not found in Xew 

 Eng-land, nor in the vicinity of the Ocean. The dis- 

 tribution of species and the characters of the shells of 

 the north find a parallel in marine moUusks, and leave 

 no doubt as to species being- speciallv adapted to 

 the station they occupy- in the first creation, thougfh 



