2 MONOGRAPH OF BRITISH LAND AND FRESHWATER MOLLUSCA. 
INTERNALLY, the ANIMAL displays the same 
general arrangements of organic structure as 
the typical //elices, a relationship shown also 
by the disposition of the retractor muscle of 
the right tentacle, which passes in its course 
between the male and female organs, this 
entanglement being indicative of a funda- 
mental unity of plan, probably dating far 
back in the history of the group, and 
differentiating it from the genus //elicella 
and its allies. 
Fic. 2.—Terminal portion of the 
right tentacular retractors in //edzx, 
showing their entanglement with the 
Characteristic features of the genus are 
the absence of the diverticulum or auxiliary —_- Reproductive organs, x 3, 
ne e ; 7 oe g.o. genital orifice; 0. retractor 
branch of the spermatheca, the frequent  .¢°ij:,¢°ommatophore; /.s. penis- 
doubling of the dart-sacs, or stylophores, the sheath; 4. lower tentacular retrac- 
» tors; 7. vagina. 
presence of accessory sacs, and the separa- : 
tion of the mucus glands from their close association with the dart sacs 
. . 5 . 
by their elevation on the vagina, though some members of the group have 
lost one or more of these organs by degeneration. 
fo) fo) 
The SHELL is usually somewhat thin, and only slightly calcified, of an 
uniformly horny or dull brownish colour, and often hispid, all peculiarities 
distinguishing species affecting a damp and shady habitat ; the aperture 
is slightly expanded and strengthened internally by a submarginal rib, 
which is most prominent basally, where denticular thickenings are some- 
times developed. 
The saw is pyenognathous and crescentic in shape, with numerous 
slightly divergent riblets on its anterior surface, which faintly denticulate 
the cutting margin, and are apparently due to overfoldings of the sub- 
stance of the jaw. 
The RADULA, as customary in the /elicidw, possesses numerous teeth, 
arranged in three longitudinal series, which comprise a median symmet- 
rical tooth, an asymmetrical bidentate lateral group, and a marginal series, 
which may become tridentate or quadridentate by the splitting of the 
ectocone of the lateral series. 
A close convergence of external characters, due to similarity of habit 
and mode of life, has led to great confusion and frequent incorrect appre- 
ciation of the status of various members referred to this group, and also a 
lamentable lack of uiiformity in their identification between the British 
and Continental authors, partially ascribable to deficient knowledge of the 
internal organization of the animals. 
I have, therefore, carefully given the names used by the authors them- 
selves for such records as I have ventured to quote, and I hope that the 
present work will clear up the confusion that still exists on this subject, 
by giving accurate figures and descriptions of the morphology and 
organization of the various species found in this country. 
Dr. Pilsbry states that the genus as understood by him is not well 
represented in the fossil state, although certain forms are known as far 
back as the Oligocene in Central Europe. 
