86 
L v 
From the original delicacy of structure of these shells, from the cir- 
cumstances under which they have become buried, and from the slight 
degree of mineralization which they have undergone, it is difficult to 
obtain their fossil remains in tolerable preservation. 
Lamarck has met with three species among the fossils of the en- 
virons of Paris : P. nitidula , P. subangulata, P. bicarinata. 
LVII. Helix. A globular or orbicular shell, with a convex or co- 
noidal spire : the opening entire, wider than long, and diminished in 
its upper part by the projection of the last turn but one of the spire. 
Shells of this genus, as well as other land and fresh-water shells, 
are rarely found in a state of petrifaction. The circumstances of con- 
servation in which they are found are, generally, such as are expli- 
cable on the supposition of their having become involved in the 
gradually accreting tufaceous matter, which is deposited by certain 
streams and rivers ; or in the stalactitic concretions forming in the 
cavities of limestone rocks, of comparatively modern formation. 
Instances of the former kind are to be met with in various parts of 
this island, as well as in numerous other parts of the world ; but the 
most remarkable instance of the latter kind is in the rock of Gibraltar, 
in which shells of this description are sometimes found. 
LVIII. Helicina. A subglobose univalve, with no umbilicus : the 
opening entire, and semi-ovate ; the columella callous, depressed at 
the lower part, and flattened. 
The analogous recent shell on which, I believe, Lamarck founds 
this genus is, trochilus, labro protenso, fasciatus , List. Hist. Conchyl. 
Plate LXI. Fig. 59. It differs from Helices in its callous columella, 
and in a little angle, which the base of the right edge forms, before 
re-uniting itself with the base of the columella. This ingenious na- 
turalist finds only one fossil shell which he can place, even with hesi- 
tation, under this genus. This incertitude, he candidly acknowledges, 
and even implies it, in the name which he has affixed to this species, 
Helicina dubia. I know of no fossil shell of this genus. 
