116 EOCENE MOLLUSCA. 
Gen. Char.—Spire sub-turbinate, depressed, or discoidal; apex obtuse; whorls 
rounded: aperture nearly circular, with a small siphon at the posterior extremity ; 
peristome simple, sometimes reflected; widely umbilicate: operculum thick, calcareous, 
formed of two laminz with a groove on the edge between them ; outer surface rather 
concave; whorls numerous, enlarging gradually, with the outer edge reflected, forming 
a spiral fringe. 
‘The genus Cyclostoma, as originally proposed by Lamarck, rested entirely on the 
circular form of the aperture, a character which applied as well to land as to marine 
and fresh-water species, and brought together animals essentially different, not only in 
their organisation, but in the structure of their shells. From this heterogeneous 
group, Draparnaud withdrew the marine species, and restricted the genus to the land 
and fresh-water species; and Lamarck afterwards formed for the marine and fresh- 
water species the genera Scalaria, Delphinula, Paludina and Valvata, and confined the 
genus to the free-air breathing land species. The animal is unisexual and operculated, 
with a proboscidiform head, furnished with two subulate annulated tentacles, oculated 
at their external bases; the respiratory opening, unlike that of the preceding sub-order, 
is largely open in front, resembling that of many of the branchiated molluscs. These cha- 
racters separate the genus and its sub-genera asa distinct group among the pulmonated 
molluscs. The modification of the organs of respiration, to which many zoologists 
have attached great importance, has been considered by others as a character of 
comparatively small value; and the resemblance which the animal of Cyc/ostoma presents 
to that of Zwrdo, in many important particulars, induced Cuvier to disregard the pecu- 
harity of the respiratory apparatus, and to place the genus in the same family as 
Turbo ; and M. Deshayes* has suggested that the Cyclostomide should form a distinct 
group near to or among the Zwréinacea. Such an arrangement, however, cannot 
consistently be adopted in any system in which the mode of respiration is admitted 
as an ordinal character; and consequently the Cyclostomide are retained, almost uni- 
versally, among the pulmonated molluscs. 
As ultimately restricted by Lamarck, the genus Cyclostoma comprised two groups, 
which presented distinct forms of the operculum; that appendage being formed, in 
one group, of a few rapidly enlarging whorls, and, in the other, of numerous slowly 
increasing whorls. Each of these groups comprised species in some of which the 
shells were more or less widely umbilicated, and in others imperforate, or nearly so. 
Montfort availed himself of the condition of the umbilicus, a character in itself of little 
generic value, and separated the widely umbilicated species under the generic name 
Cyclophorus, retaining the imperforate species for his genus Cyclostomus; but the 
characters presented by the opercula were altogether overlooked or disregarded. 
Each genus, therefore, comprised species presenting different forms of operculum ; 
* Lam. Hist. naturelle, 2d edit., vol. viii, p. 351. 
