THE MOLLUSCA 



985 



51 (52) Shell with body- whorl usually inflated. Inner edge of mantle digitate 

 or lobed, extending partly over the shell. 



Physa Draparnaud. 



T The species of this genus are very numerous and extremely variable, so that 

 many more species have been described than really exist. The Physae are found 

 in all parts of the country, but the majority of the species are inhabitants of the 

 Northern States and Canada. Example, P. gyrina Say, (Fig. 1438; X ii)- 



FIG. 1438. 



52 (51) Shell slender, elongated. Inner edge of mantle simple, not extending 

 over the shell Aplexa Fleming. 



The typical species, A. hypnorum L., (Fig. 1439; X ii), is circumboreal and has a 

 general distribution through the Northern States and Canada from the Atlantic 

 to the Pacific. 



53 ( 2 3) Shell patelliform or depressed, dextrally spiral, neritiform or planorbi- 



form. Animal sinistral or dextral; tentacles short, blunt, 



cylindrical Family ANCYLIDAE . . 54 



Five genera: 54, 59, 60, 61, 62. 



54 (59) Shell patelliform, small, thin; apex posterior, slightly inclined to one 



side Ancylus Miiller . . 55 



55 (58) Apex inclined to the right Subgenus Ancylus s.s . . 56 



56 (57) Apex subacute, radially striate Section Ferrissia Walker. 



Numerous species and of general distribution, usually found adhering to 

 stones, etc., in running water. Type, Ancylus rivularis Say, (Fig. 1440; X 3). 

 .Fie. 1440. 



57 (56) Apex depressed, smooth Section Laevapex Walker. 



The species of this group are usually found in quiet water and are, as 

 a rule, larger, thinner, and more depressed than the Ferrissias. Type, 

 Ancylus diaphanus Hald., (Fig. 1441; X 2f). 

 FIG. 1441. 



58 (55) Apex inclined to the left Subgenus Acroloxus Beck. 



Only one American species, Ancylus nuttattii Hald., (Fig. 1442; X 2), 

 from Oregon, has been referred to this group, but its anatomy is un- 

 known and its generic position is very doubtful. 



FIG. 1442. 



