60 TRACHELIPODA. CYCLOSTOMA. 
are exceedingly thin and delicate; and are at once distinguished from the 
Lymnza, by the convolutions being reversed. ; 
Genus LXX VII. — PLANORBIS, — Miller. 
Generic Character. — Shell discoidal, umbilicate ; spire 
and base depressed; apex always distinct; the volutions 
turning nearly on the same plane, from right to left, so that 
when the spire is held upwards, and the aperture next the 
observer, it is situate on the left hand side ; volutions ven- 
tricose in many species, often carinated, either above or 
below; aperture entire, obliquely semilunate, its length and 
breadth being nearly equal, but broader than long in some 
instances ; outer lip sometimes thickened; umbilicus very 
wide. Destitute of an operculum. 
Planorbis rotundatus. ‘Plate V. fig. 4. Found in the 
London Clay, at Hordwell Cliff. 
Section 1. Volutions not carinated, rounded both above 
and below ; the spire flattened, and slightly concave. Ex- 
ample, P. rotundatus. 
Section 2. Carinated, volutions flat above. Example, 
P. marginatus. 
Section 3. Pellucid, spire deeply umbilicate ; volutions 
slightly carinated, and lenticular. Example, P. nitidus. 
The upper side or apex of the shells of this genus, is that in which the 
volutions terminate in the centre, generally in 4 point, with the aperture next 
the left hand ; the umbilicus will be found on the opposite side. 
The shells of this genus are found in stagnant waters, ditches, lakes, and 
also in slow running streams. Their geographical range seems limited to 
Europe and America, and those of the latter country have a thickened lip. 
Fossil species are found in the fresh water formations at Paris, and the Isle 
of Wight. 
TRIBE 1X.— COLIMACEA. 
_ Shell spiral ; external surface smooth, exhibiting only the 
lines of growth; right margin of the aperture frequently 
reflected outwards ; animals terrestrial, with cylindrical 
tentacula, some species with an operculum, and others 
devoid of one. 
SUB-DIVISION I. — Animals with two’ tentacula. 
Genus LXXVITI.— CYCLOSTOMA. — Lamarck. 
Generic Character, — Shell thin, turbinated, variable in 
shape, the apex in most species obtuse, and the volutions 
ventricose ; aperture entire, circular, or nearly so, in the 
