186 Couthouy's Description of a 
near the base, which forms a sub-angular sinus or inden* 
tation 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 ex- 
tending 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 occa- 
sional shades of a yellowish white, in old shells. 
Height ten-fortieths, diameter of last whorl eleven- 
fortieths of an inch, 
Inhabits the rapids of the. i Potomac, Va. 
My own collection, that of the Bost. Soc. Nat. Hist. " 
A. A. Gould, M. D., and several others in Boston. 
Osservations. This shell, at first sight, might be 
mistaken for AwcurLorvs monodontoides, Conrad, from 
Alabama, but may be distinguished from it by the pecu- 
liar 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 re- 
sembles à plait or fold at that part; whereas in ours, it 
is formed by an. oblique, inward projection of the co- 
lumella near the base. The external conformation is 
exceedingly irregular, varying from sub-conical to glo- 
bose, sometimes compressed on the back, at others 
strongly gibbous. ‘The aperture is also frequently dis- 
torted. Young specimens are of a light, olive-green 
color, while older ones are nearly black, and usually 
covered with an earthy coating. The lower whorl is 
invariably marked at its base with a broad, dpsk- row 
