THE MOLLUSCA OF COLORADO 87 



Superorder Streptoneura 

 Order Ctenobranchiata 

 Suborder Streptodonta 

 Family Valvatidae 



Genus Valvata, 1 sp. 



Key to Colorado Mollusca 



Inherent difficulties in the description of shells make it almost impossible to formulate 

 a key which is sufficient of itself for the identification of species, but this key may be of 

 assistance to students, who, however, must in the end, depend upon more extended descrip- 

 tions, in other works. The great majority of our species are described in Baker's Mollusca 

 of the Chicago Area, published by the Chicago Academy of Sciences; Binney's "A Manual 

 of American Land Shells," Bulletin No. 28 of the United States National Museum, Binney, 

 Bland and Tryon's "Land and Fresh-Water Shells of North America," Parts I, II, III, 

 and IV, Smithsonian Miscellaneous Collections, Nos. 143, 144, 194, and 253, respectively; 

 Prime's "Monograph of American Corbiculidae." Smithsonian Miscellaneous Collections, 

 No. 145. 



PELECYPODA (Bivalves) 



A. Shell more than 2 inches in length, lateral teeth posterior to cardinal teeth. 

 a. Rather thin shelled, cardinal teeth slightly or not at all developed. 



1. Coarse wrinkles on beaks in parallel double loop. 



Anodonta grandis giganteus 



2. Coarse parallel wrinkles on beaks, not doubly looped. 



Strophitus edentulus pavonius 



3. In addition to coarse wrinkles, fine ridges on umbones, directed posteriorly. 



Anodontoides ferussacianus 

 h. Shell solid, cardinal teeth well developed. 



1. Length about twice the height or more. 



aa. Beak wrinkles 5 to 7, in long poorly defined double loop, cardinal teeth 



double in both valves. Lampsilis anodontoides 



hb. Umbones with about 14 fine undulating ridges. Lampsilis luteola 



cc. Umbones with 5 to 8 coarse concentric ridges. Unio tetralasmus camptodon 



2. Length about J greater than height, 6 coarse beak wrinkles not doubly looped, 

 cardinal teeth double only in left valve. Lampsilis ventricosa 



B. Shell less than one inch in length, lateral teeth both anterior and posterior. 



a. Animal with one short siphon; mussel generally small to minute, i .5 to 8 mm. long, 

 in a few species 8 to 1 2 mm. ; anterior part generally longer than posterior. 



Pisidium spp. 

 h. Animal with 2 longer siphons, connate at their bases for a longer or shorter distance ; 

 mussel averaging larger, 5 to 20 mm. long; anterior part generally shorter than 

 posterior. 

 I. Shell and hinge generally stouter, beaks rarely calyculate (capped). 



Sphaerium spp. 



