THE MOLLUSCA OF COLORADO 79 



supposed barriers, is considered an important field for extensive and 

 intensive collecting and study of mollusks. Up to the present time the 

 study of the influence of altitude upon mollusks in this region has given 

 chiefly negative results. Instead of dwarfing the species as in Montana 

 and other regions where the cold of high altitudes is more intense,^ the 

 higher altitudes seem more favorable to land snails than lower levels, 

 because of increased humidity. The finest specimens of Oreohelix 

 strigosa and Vitrina alaskana found have been at ii,ooo and 9,300 feet 

 respectively. IngersolP reports the same fact concerning Thysanophora 

 (Microphysa). On the other hand, the larger fresh- water snails we have 

 found do not occur in the lakes at timber line at aU. Among bivalves 

 we have found Pisidia at 11,000 feet, Calyculina at 8,500 feet, but no 

 UnionidcB at all in the mountains or anywhere in the western half of the 

 state. The only thick-shelled Unionidos found in the state are those 

 from Lodge Pole Creek and Las Animas County, though a thin- shelled 

 species (Strophitus edentulus pavonius) is found close to the foothills. 



The majority of our species, both land and fresh water, are also found 

 in the Mississippi Valley and eastward, probably having reached Colorado 

 from the eastward or northward, but, on the other hand, there is an im- 

 pressive and probably significant absence of such genera as Polygyra, Cam- 

 peloma, Viviparus, Goniohasis and Pleurocera, which are so well repre- 

 sented in the eastern states. The distribution of Oreohelix and Thysano- 

 phora indicates that they have crossed the mountains from the westward; 

 but if so, the former must have arrived at the base of the foothills long ago, 

 as suggested by fossil specimens in the Quaternary deposits. Viviparus, 

 Campeloma and Goniohasis lived in northeastern Colorado, together 

 with Anodonta, Unio, and Physa, during the latter part of Cretaceous 

 time, as shown by the Laramie fossils of Crow Creek and neighboring 

 localities, but the three first-named genera have disappeared from the 

 state, while Physa is among our most abundant mollusks at the present 

 time. Anodonta is reported from Lodge Pole Creek, its near relative 

 Strophitus is our most common large bivalve, and Unio has been found 

 in two localities. Lymncsa, Planorbis, and Unio occur in Jurassic strata 



' Morton J. Elrod, Bull. Univ. Mont., No. 17, p. 259. 

 ' Eighth Ann. Kept. Hay den Survey, p. 398. 



