PULMONATA. 69 
a change in the ordinary conditions of temperature, and of the nature and supply of 
food, will be attended with appreciable differences in the development, although not in 
the organisation, of the animal; and that these differences will be represented in, and 
will modify the form of the shell. And to such a cause, perhaps, may be attributable 
the distinction, trivial as it is, which, as we have seen, exists between the shell of 
the living H. labyrinthica and those of its Eocene representatives. 
The identity in question exhibits an instance of a terrestrial species surviving 
important geological changes, and prolonging its existence through geological epochs 
of very great extent, but to the probable duration of which no approximation even can 
be made and yet preserving its normal form almost without modification ; an instance 
unparalleled, if, as will probably prove to be the case, the various forms of Terebratula 
referred to the recent 7. caput-serpentis belong to different species. 
Brogniart, ((Ann. du Muséum d Histoire Naturelle,’ tom. xv, p. 380,) has de- 
scribed a small trochiform Helix from the neighbourhood of Mans (//. Menard), 
which, in the general character of its lineation, resembles this species. It is, however, 
larger ; and the whorls, although described as being “nearly equal,” appear, from the 
figure given, to enlarge more rapidly. The aperture is neither described nor repre- 
sented, and it is impossible, therefore, to form any opinion as to the identity of the 
shell with the present species. 
Size.—Diameter, 1-10th in.; elevation 1-10th in. 
Localities.—Hordwell Cliff ; Headon Hill. 
No. 21. Heurx sus-LaByrintnica. F. 2. Rdwards. Tab. XI, fig. 4 a—e. 
Hf. testd minimd, globoso-conicd, umbilicatd; spird elevatd, apice obtuso: anfractibus 
sex, rotundato-convexis, gradatim majoribus, transversim lineatis : aperturd, obliqua, semi- 
lunari, simplict (2); umbilico parvo. 
I possess only one specimen, and that merely a cast, of this small and very rare 
Helix. Although more pupiform than /Z. /abyrinthica, it approaches so nearly to that 
shell that I feel great hesitation in referring it to a distinct species; on examination, 
however, differences appear which scarcely justify my describing the shell as merely a 
variety. 
It is a small, globosely conical shell, with an obtuse apex, and formed of six 
roundedly convex whorls, increasing in size very slowly. The impression of the 
whorls in the matrix presents a faint lineation, too regular to be due to lines of 
growth merely. The aperture is oblique and semilunar, but is too imperfect to enable 
me to say whether the peristome was or was not thickened or reflected. On the 
outer lip of the penultimate whorl are two linear impressions similar to those produced 
by lamelliform teeth, to the presence of which they may, perhaps, be attributed; but 
