THE MOLLUSCA 977 



regular intervals by vascular junctions (Eulamellibranchia), or are 

 entirely absent (Septibranchia). 



All of the North American fresh-water lameUibranchs 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 " EulameUibranchia, in which the mantle 

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

 Siphons absent or very short. GUIs 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 faiuia: 



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 . . a 



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



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



pHcate at the base; outer lip usually dentate or Urate. 



Family Aueiculidae . . 4 



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



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



Carychium M tiller. 



A group of small spedes of general distribution from the Atlantic to tlie 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. 140S.' 



• Unless otherwise indicated, the figures are ot natural size. In other cases, the 

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

 apecies, divide the length- of the figure in millimeters by the index. 



