GASTEROPODA PECTINIBRANCHIATA . 57 



Cyclostoma, Lam.* 



The Cyclostomne sliould be distinguished from all the others be- 

 cause they are terrestrial, as instead of branchiae, the animal has 

 merely a vascular network spread over the parietcs of its pectoral 

 cavity. In every other respect, however, it resembles the other 

 animals of this family ; the respiratory aperture is formed in the 

 same Avay above the head by a great solution of continuity ; the 

 sexes are separated; the penis of the male is large, fleshy, and re- 

 flected into the pectoral cavity ; the two tentacula are terminated by 

 blunt tubercles, and two other tubercles, placed on their external 

 base, support the eyes. 



The shell is a spiral oval, with complete whorls, transversely and 

 finely striated, and its aperture, in the adult, is surrounded with a 

 small ridge. It is closed by a small round operculum. Found in 

 woods, under moss, stones, &c. 



The most common is the Turbo elegans. List., 27, 25, about 

 six lines in length and of a greyish colour ; found under all the 

 mossesf. 



Valvata, Mull. 



The Valvatae inhabit fresh water ; their shell is convoluted in 

 almost one plane like that of a Planorlns, but the apertiu-e is round, 

 and furnished with an operc\ilum ; the animal, Avhich has two slen- 

 der tentacula, with the eyes at their anterior base, respires by means 

 of branchiae. In a species fovmd in France, 



Valv. cristata,Mv\\\.; Drap., I. 32,33; Gruet-Huysen, Nov. 

 Act. Nat. Cur. X, ^\. xxxviii, the branchiae, formed like a 

 feather, project from under the mantle and float externally, vi- 

 brating with the breathing of the animal. On the right side of 

 the body is a filament which resembles a third tcntaculum. The 

 foot is divided, anteriorly, into two hooked lobes. The penis 

 of the male is slender, and reflected into the branchial cavity. 

 The shell, Avhich is hardly three lines broad, is greyish, flat, 

 and umbilicated. Found in stagnant waterij:. 

 It is here that we must place tlie completely aquatic shells, or 

 those respiring by branchia;, which belonged to the old genus Helix ; 

 i. e., those in which the penultimate whorl forms, as in the Helices, 

 Lymnaeae, &c,, a depression which gives the aperture more or less of 

 the figure of a crescent §. 



The three first genera are still closely allied to Turbo. 



* The Cyilostoiiue and the Helicines form the order of the Pulmonea Opercu- 

 XATA of M. dc Ft'nissac. 



t Add, Turbo lincina, List., 26, 24 ,—T. htbco, List., 25, 23 ;— T. diibiiis, Born., 

 XIII, 3, 6 ;— r. Hmbaii'S, Chemn., IX, cxxiii, 1075. 



We should distinguish, among the fossils, the Ci/dostonia muniia of Lam., Brongn,. 

 Ann. du Mus., XV, xxii, 1. 



+ Add, Vahata planofbis, Drap., I, 34, 35;—!'. minuta. Id., 36—38. 



§ ITiey constitute the Ellipsostoma of r.l, de Blainviile. 



