54 HELICODONTA OBVOLUTA. 
superiority in dominating power of any species and show that the facts of 
geographical distribution as proof of dominance must always receive the 
consideration due to their undeniable importance. 
Dr. Germain regards as the ancestral form the H. bernardi Michaud, a 
much smaller shell, of about 5 mill. in diameter, with a more vertical 
aperture, which replaces H/. obvoluta in the diluvial clays of Combe-de- 
Clary, near Hauterive, Dréme. 
It was first discovered in this country in Ditcham Wood, Hampshire, 
and added to the British lst in 1831, by Dr. Lindsay, in a communication 
to the Linnean Society. 
Diagnosis.—H. obvoluta cannot be confused with any other British 
species, the depressed and somewhat concave spire, and the more or less 
distinct indications of the apertural denticles preclude possibility of error. 
INTERNALLY, the differences from other British species are equally 
striking, the degree of degeneracy exhibited by the organs assumed to 
represent the mucus-glands and dart-sac being amply sufficient to satis- 
factorily determine the species. 
Description—The ANIMAL is slender, of a dark leaden-grey colour, with large 
and moderately distinct rugze, which do not present any readily perceptible arrange- 
ment ; DORSAL GROOVES distinct, and enclosing a single row of ELONGATE 
TUBERCLES; there is no trace of facial furrows, and a mere indication of LATERAL 
GROOVES shown by the paler colouring of the area beneath their position, the 
tubercles of this paler portion appear of a darker grey against the pale ground 
colour; the TAIL is narrow and pointed, and does not extend beyond the margin of 
the shell; MANTLE pale greyish, speckled with a darker shade ; OMMATOPHORES 
long and slender, semitransparent, and showing the dark RETRACTORS throughout 
their length, and giving a darker shade to each side of the back; the MUCUS 
is colourless and thin, but not very plentiful. The shell is carried almost horizon- 
tally, and slightly inclined to the right. 
SHELL discoidal in shape, flat or slightly coneave above, convex beneath ; 
WHORLS 64, increasing slowly in size and deflected at the aperture when adult, 
laterally compressed, but bluntly angu- —— 
late above, near the suture, which is ° 
thus deeply channeled ; EPIDERMIS 
thick, opaque warm-brown in colour, 
bearing obliquely undulate striz and 
beset by a number of smooth, stiff, 
caducous and slender whitish hairs, furnished with several knobbed exerescences 
along their length, sometimes bent at the tip, or even almost prone and usually 
inclined to be recurved or directed in the opposite direction to those of H. hispida; 
they are arranged in oblique rows, which run in an almost opposite direction to 
the lines of growth, and are borne upon indistinct 
protuberances, which are said to be arranged 
somewhat quincunxially, but do not all support f 
hairs. APERTURE small and obliquely triangular, } ; if 
inflected above ; OUTER LIP very thick, reflected, i 
and obscurely toothed within on the basal and { t i 
palatal margins, of a purplish colour, which, how- P } 
ever, is very fugitive and quickly fades on the i a6 1 
death of the animal; there is also a distinet con- ' \ \ \ 
striction or groove behind the lip, especially deeply A a \ A 
Fic.79— Helicodonta obvoluta (Miill.)after Sandberger. 
sunk behind the palatal tooth-like thickening. The 
UMBILICUS is openly convoluted, almost tubular, 
and exposing all the internal spire. 
Diam., 12 mill. ; alt. 5 mill. 
Fic. 80.—Periostracal hairs from 
the umbilical region of H. obvoluta, 
from a highly magnified drawing by 
the late Mr. G. Sherriff Tye. 
The REPRODUCTIVE ORGANS are, according to Moquin-Tandon, thick and white 
and aggregated in a trilobed form; the HERMAPHRODITE DUCT is thick and some- 
what convolute, but attenuate at each extremity ; the ALBUMEN GLAND is small, 
thin, of a yellowish colour, irregularly lobulated and curiously bent over at the tip; 
