CHAP. VI. SUB-GENERA OP CYCLOSTOMA. 185 



furnished with an operculum. Nothing, perhaps, 

 can better exemplify the artificial nature of Cuvier's 

 Pectinibranchia, than his placing Helicina between the 

 two fluviatile genera Ampullaria and Melania, and Cyclo- 

 stoma between Scalaria and Valvata ; in both instances 

 thrusting in a group of land-shells between two others 

 which only inhabit water. This is the more inexcusable, 

 because it did not originate in an ignorance of the ani- 

 mals j and yet he observes that the Helicina;, "judging 

 by the shell, are Ampullaria, in which the margin of the 

 aperture is reflected."* The only reason assigned for 

 placing Cyclostoma after Scalaria, is " because the aper- 

 ture is entire, nearly or quite round, and operculated." t 

 This is quite true, but every student will readily per- 

 ceive that this is merely a remote analogy. 



(171.) The genus Cyclostoma, like the last, has 

 the sexes distinct ; but this is the only one character 

 in which they differ from the other pulmoniferous land 

 and river snails. The passage between this and the last 

 group is rendered unquestionable by such shells as Heli- 

 cina elegans Gray, which has the characters of both 

 united. The typical Cyclostomce are spiral shells, the 

 last whorl being but little larger than that which 

 precedes it. The orifice of the mouth is circular, 

 and is closed by a horny operculum. The pillar is 

 often wanting ; but this variation occurs in species 

 so close to each other, that it cannot be considered 

 a sub-generic character. They are usually found in 

 dry arid situations: the island of Malta abounds with 

 them, where thousands may be gathered on the scanty 

 herbage of the rocks; and many elegant species oc- 

 cur in the mountains of Jamaica. On coming to the 

 aberrant sub-genera, we find great diversity in the form, 

 although none in the aperture. In Cyclophora, the shells 



* Cuvier further remarks that the organs of respiration are arranged as 

 in the Cyclostomce, and, like the latter, they can live out of water. That 

 these two genera are naturally united, both by the animal and the shell, is 

 thus admitted, while we can only say that they are both as terrestrial as 

 the garden snail ; they cannot, in fact, live out of the air, as we always 

 killed the animals of our specimens by plunging them into water, 



t Griff. Cuv. xii. p. 58. 



