224 INVERTEBRATA OF MASSACHUSETTS. 



It is not yet satisfactorily determined under what family this genus 

 should be arranged. Its animal is closely allied to the LIMNEANA, 

 and its natural relations are certainly stronger to this family, notwith- 

 standing the form of the shell, than to the CALYPTRACEA, where it 

 has usually been placed. 



A'NCYLUS RIVULARIS. 



Shell elongated-oval, sides rectilinear, apex nearer to one side, 

 nearly central. 



FIGURE 153. 



State Coll., No. 281. Soc. Cab., No. 842. 



Ancylus rivularis, SAY; Nicholson's Encyc. (Amer. ed.) t Art. Conckology, 

 Journ. Acad,. Nat. Sc., i. 125. 



Shell small, narrow, elongated-oval, the sides nearly parallel, 

 but one end is somewhat narrower than the other, and both are 

 regularly rounded ; apex nearly equi-distant from both extremi- 

 ties, nearer to, and leaning to, one side and one end ; aperture 

 oval ; color dark-green. Length 1 inch, breadth ^ inch. 



Found on stones and floating leaves in rivulets and ponds. 



It is closely allied to A.JluvidtiUs of Europe ; but the apex is less 

 acute and more central. There is another American species, the A. 

 tardus. Say, which has been found by Professor Adams in Vermont, 

 but which I have not yet found in this State. It is much more round- 

 ed and conical than this, and the apex is not lateral. 



A'NCYLUS FUSCUS. 



Shell oval, depressed, convexity regular, not compressed laterally, 

 curvilinear at the sides ; apex obtuse, a little to the right and rear 

 of the centre ; epidermis coarse, brown, surpassing the margin. 



FIGURE 152. 

 State Coll., No. 283. Soc. Cab., No. 2383. 



Incylus fuscus, ADAMS ; Bost. Journ. Nat. Hist., iii. 329, pi. 3, f. 17. 



Shell small, very thin and pellucid, of a rounded oval form, the 

 entire outline regularly curved ; depressed and regularly convex, 

 not compressed at the sides ; apex slightly elevated, bluntly 



