ANCULOSA. 393 



^^•ith monodontoides is A. dentatus^ Couthuoy, a description of 

 which follows : — 



Anculotus dentatus. — Animal much like that of Melania ; foot broad, 

 short, rounded and thick; body and head black, the latter subor- 

 bicular, terminating in a short, proboscidiform mouth, and furnished 

 with two short, rather stout and pointed tentacula, black, posteriorly 

 and with faint, grayish, transverse bands on their anterior side ; eyes 

 minute, situated on a slight enlargement of the tentacula near their 

 external base. 



Operculum elongated, unguiforra, thick, corneous, blackish or 

 brown, opaque ; spire terminal, increment, coarse and apparent. 



Shell rounded or obtusely conical, subdiaphanous, very irregular 

 in its conformation, frequently gibbous and distorted; the color 

 varies from light olive-green to black, according to the age of the 

 specimens; whorls five or six in number, the last constituting the 

 greater portion of the shell, very much inflated and ventricose, and 

 sometimes ornamented with two or three dax'k brown, transverse 

 bands ; spire obtuse, always considerably eroded, unless in very 

 young shells ; incremental stria3 oblique, in some rig. 772. Fig. 771. 

 individuals barely apparent, and in others forming 

 strong ridges on the last whorl ; aperture rounded, 

 effuse at the base ; right lip thin, sharp and broadly 

 everted; columella dark brown or purple, flattened, 

 strongly arcuated, with a dentiform projection near the base, which 

 forms a subaugular sinus or indentation below it. • Adjoining the 

 columella is a strongly marked lacuna or fossa, most conspicuous in 

 very old shells, but apparent in every stage of growth, and extending 

 from the base of the shell to the centre of the lower whorl. There 

 is no umbilicus, properly speaking, that region being consolidated 

 by the columella. The internal color is chiefly greenish or brownish? 

 with occasional shades of yellowish-white in old shells. 



Habitat. — Inhabits the rapids of the Potomac Eiver, Virginia. 



Height, ten-fortieths ; diameter of last whorl, eleven-fortieths inch. 



Observacions. — This shell at first sight might be taken for Ancu- 

 lotus monodontoides, Conrad, of Alabama, but may be distinguished 

 from it by the peculiar flattening of the columella, which is deep 

 purple or brown instead of white, and the remarkable fossa in the 

 umbilical region. In that species, moreover, the tooth is situated on 

 the middle of the columella and resembles a plait or fold at that part 



