Mr. Lightfoot’s Account, ice. 161 
we are authorifed, by the difcovery of many more of a fimilar 
ftrufture, to rank it under a new genus. 
That I may give a more full and fpecific defeription of this 
lingular (hell, it mud be obferved, that its figure is a flatted 
fpiral, umbilicated on one fide, convex on the other, but' yet 
llightly depreffed in the centre, meafuring in diameter about a 
quarter of an inch ; that it is generally coiled up into four volu- 
tions, which are convex above, and fo nearly plane beneath as 
to form an acute or carinated margin ; and that each of thefe 
volutions, on the upper fide, has a narrow thread-like border 
or fillet on the interior edge. The front view of the mouth is 
obliquely femioval, the upper edge projecting farther than the 
lower. 
The fubftance of the {hell is very brittle and pellucid, and, 
when alive, of a reddifn brown or chefnut colour throughout, 
except about three or four faint white lines, which appear like 
rays running from the central umbilicus to different and nearly 
equidiftant parts of the circumference. Thefe white lines are 
not ftraight, but (haped each like a fliort curve, or comma, on 
the upper fide, and are nothing elfe but the (hades of the fepta 
in the cavity of the fhell. 
Such is its external appearance. The internal ftru&ure is 
extremely curious ; for the whole cavity is divided into three, 
four, or five chambers or compartments (according to the age 
of the fhell) at nearly equal diftances, by tranfverfe fepta of a 
hard white brittle femipellucid fubftance,, refembling agate or 
enamelled glafs. Each of thefe fepta has a triradiated aper- 
ture not unlike the Greek capital upfilon, or the Roman Y, 
inverted, (jf) through which the animal, by means of its foft 
compreffible and extenfible nature, eafily contrives to extrude 
Vol.LXXVI. Y itfelf, 
