THE MOLLUSCA 977 



regular intervals by vascular junctions (Eulamellibranchia), or are 

 entirely absent (Septibranchia). 



All of the North American fresh-water lamellibranchs belong to 

 the order Eulamellibranchia. This order is divided into nine sub- 

 orders, of which only one is represented in our fauna. The suborder 

 Submytilacea consists of " Eulamellibranchia, in which the mantle 

 is only slightly closed, generally there is only a single suture. 

 Siphons absent or very short. Gills smooth. Nearly always dimy- 

 arian (with two adductor muscles). Shell equivalve with an exter- 

 nal ligament." 



The Submytilacea are divided into a large number of families 

 of which seven are represented in the North American fauna: 



Margaritinidae Sphaeriidae 



Unionidae Cyrenidae 



Dreissensiidae Cyrenellidae 

 Rangiidae. 



KEY TO NORTH AMERICAN FRESH- AND BRACKISH-WATER 



MOLLUSCA. 



1 (103) Animal with a distinct head, bearing, usually, contractile tentacles. 



Shell univalve Class Gastropoda . . 2 



2 (63, 100) Animal inoperculate, pulmoniferous. . Order Pulmonata . . 3 



3 (17, 22) Animal terrestrial or semiamphibious. Shell spiral, columella 



plicate at the base; outer lip usually dentate or lirate. 



Family AURICULIDAE . . 4. 



4 (9) Foot entire, not divided transversely. . Subfamily AUEICULINAE . . 5 



5 (6, 7) Shell minute, pupaeform, outer lip thickened, reflected, or expanded. 



Carychium Miiller. 



A group of small species of general distribution from the Atlantic to the Pacific. 

 The only genus in the family found at a distance from the seashore. Found in 

 damp places under dead leaves, pieces of bark, etc. They are usually included 

 among the terrestrial species and are included here rather on account of their 

 systematic position than as belonging strictly to the fresh-water fauna. Example, 

 C. exiguum Say, (Fig. 1405; X 10), from the Eastern States, 

 FIG. 1405 - 1 



1 Unless otherwise indicated, the figures are of natural size. In other cases, the 

 amount of enlargement or reduction is indicated. To obtain the actual size of any 

 species, divide the length of the figure in millimeters by the index. 



