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 laminae 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, Delp/iinula, Paludina and Vahata, 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 as a 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 Cyclostoma presents 

 to that of Turbo, in many important particulars, induced Cuvier to disregard the pecu- 

 liarity of the respiratory apparatus, and to place the genus in the same family as 

 Turbo ; and M. Deshayes* has suggested that the Cyclostomidte should form a distinct 

 group near to or among the Turbinacea. 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 Cyclostomida 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. 



