86 



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 environs 

 of Paris : P. nitidula, P. subangulaia, and P. bicaiinata. 



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 conserva- 

 tion in which they are found are, generally, such as are explicable on 

 the supposition of their having become involved in the gradually acre- 

 ting 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. Helidna. A subglobose univalve, with no umbilicus : the 

 opening entire and semi-ovate ; the columella callous, depressed at the 

 lower part, and flattened. 



The analagous recent shell on which, I believe, Lamarck founds 

 this genus is trochilus, labro protenso, fasciatus, List. Hist. Conchy I. PL 61, 

 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 naturalist finds only one 

 fossil shell which he can place, even with hesitation, under this genus. 

 This incertitude, he candidly acknowledges, and even implies it, in the 

 name which he has affixed to this species, Helidna dubia. 1 know of no 

 fossil shell of this genus. 



