420 
MOLLUSCA. 
Ahera Jlexilis. — The Flexible Aker a. 
Plate LXXXVII. fig. 29. 
Oval, membranaceous, excessively thin and flexible; horn- 
coloured, with a convolute truncated channelled spire ; pillar 
lip a little thickened and white. Three-fourths of an inch long. 
Inhabits the British seas. 
Genus 12.—DIA PH ANA.— Brown. 
Generic. Character. —Animal unknown. — Shell small, spiral, 
fragile, somewhat orbicular; aperture encompassing the body, 
wide ; pillar lip slightly reflected, behind which is a subumbili¬ 
cus ; spire very short, never exceeding two volutions, and hardly 
rising above the body, which is concave at top. 
Diaphana Candida . — The White Diaphana. 
Plate LXXXVII. fig. 30. 
Pure white, ventricose; aperture whole length of the shell, 
narrowed at top ; spire short; subumbilicated. One-sixth of 
an inch long. Inhabits the Frith of Forth. 
Genus 13.—A CERA.— Lamarck. 
Generic Character. —Body ovate, convex, transversely divided 
above into two parts ; the foot with dilations in the form of 
wings below ; head indistinct; branchiae situated on the back, 
greatly behind, and covered by a mantle ; destitute of a shell. 
TRIBE IV.-CALYPTRACEA. 
The branchial are situated in a dorsal cavity near the neck, 
and included in the cavity or projecting beyond it; shell invari¬ 
ably exterior. 
Genus 14.—ANCYLUS.— Lamarck. 
Generic Character .—Body creeping, enveloped in the shell; 
two compressed subtruncated tentacula, with the eyes situated 
at their internal base; foot short, elliptical ; somewhat narrower 
than the body; shell thin, obliquely conical; aperture oval, with 
a pointed apex, which is inclined backwards; margins simple. 
