336 MOLLUSCA. 



Onchidium, Buchan.(l) 



A broad, fleshy mantle, in the form of a shield, overlapping the foot 

 at all points, and even covering the head when it contracts; two 

 long retractile tcntacula, and on the mouth an emarginated veil, 

 formed of two triangular and depressed lobes. 



The anus and respiratory orifice are under the posterior edge of 

 the mantle, where, a little more deeply, we also find the pulmonary 

 cavity. Close to them, on the right, opens the female organ of ge- 

 neration, that of the male, on the contrary, is under the right great 

 tentaculum, the two openings being united by a furrow, which ex- 

 tends along the under part of the whole of the right margin of the 

 mantle. These animals, destitute of jaws, have a muscular gizzard, 

 followed by two membranous stomachs. Several of them inhabit 

 the sea-shore, but in places where the ebb leaves them uncovered, 

 so that they can readily breathe the natural air(2). 



The aquatic Pulmonea, with complete shells, were also 

 placed by Liimaius in his genera Helix, Bulla and Voluta, 

 from which it has been found necessary to separate them. 



In the first were comprised the two following genera, where 

 we find the internal edge of the aperture crescent-shaped, as 

 in Helix. 



Planorbis, Brug.(3) 



The Planorbes had already been distinguished from the Helices by 

 Brugieres, and even previously by Guettard, on account of the slight 

 increase of the whorls of their shell, the convolutions of which are 



(1) OxciiiDiux, a name given to this genus because the first species {OncMdiunv 

 iypha., Buchan., Lin. Soc. Lond., V, 132) was tuberculous; I now know one that 

 is smooth, the Onchidiuvi laevigatujn, Cuv., and four or five that are tuberculous: 

 Onch. Peronii, Cuv., Ann. du Mus., V, 6; — Onch. Slouniiy Cuv., Sloane, Jam., pi. 

 273, 1 and 2; — Onch. vemiculatum, Descr. de I'Eg., Moll. Gaster., pi. ii, f. 3;— 

 Onch. celticum, Cuv., a small species from the coast of Brittany. 



N.B. M. de Blainville has changed the name of Onchidium into that of Peiionia, 

 and applied the former to the Vaginulae. These Peroni^e he places among his 

 Ctclobrais-chiata, but I can see no real difference between their respiratory 

 organ and that of the other Pulmonex. 



(2) See Cbamisso., Nov. Act. Nat. Cur., XI, part I, p, 348, and Van Hassel, 

 Bullet. Univers., 1824, Sept., Zool., 83. 



(3) Hel. vortex; — H. coi-neu; — H. spirorbis; — H. polygyru; — H. contortUi — // 

 nitidai — H. alba; — H. similis. 



