MOLLUSCA 



137 



quite large. These were allied in a general way to those now 

 inhabiting the Mississippi Valley, where may still be found genera 

 of molluscs and fishes long ago extinct in Colorado. The eleva- 

 tion of the land drained off the waters, and created conditions 

 unsuitable to the continued existence of many forms of life. 

 Eastward, the land indeed rose out of the sea. but great rivers 

 and bodies of standing water were favorable for the survival of 

 ancient types of freshwater life. Possibly the dinosaurs survived 

 longer than we think in these regions. The great lakes of the 

 north, unlike Lake Baikal in Siberia, have not conserved the 

 relics of past times. They were formed in a very recent geological 

 epoch, and are relatively speaking of modern origin. 



Our pulmonate water snails are much more numerous, and 

 some of them quite interesting. They belong to a few genera, easily 

 recognized. Planorbis (the name meaning flat circle) is coiled 

 in one plane, though the whorls may be quite broad. Lymnaea 

 is suboval or conical, with a pointed apex, and the aperture is 

 to the right when the shell is held with it in view. Physa is not 



Lymnaea 



bulimoides 



Lea 



Physa 

 virgata 

 Gould 



Lymnaea stagnalis 

 appressa Say 



Ancylus hendersoni Walker 

 (after Walker; enlarged) 



1-^ 



Ferrisia 



rivularis 



Say 



Aplexa 

 hypnonim 

 Linnaeus 



unlike Lymnaea, but the aperture is to the left, and the shell is 

 said to be sinistral. Many authors separate from Physa the 

 genus Aplexa, which has a narrow, highly polished shell. Our 

 only species (A. hypnorum of Linnaeus) is also common in Europe. 

 The remaining type is like a little limpet, though not at all related 



