GASTEROPODA PULSION EA. Ol 



PULMONEA AQUATICA, 

 Have only two tentacula, as already stated ; they are continually 

 compelled to rise to the surface for the purpose of breathing, so 

 that they cannot inhabit very deep water; thoy are usually found 

 in fresh water or salt ponds, or at least in the vicinity of the sea- 

 coast and of the mouths of rivers. Some of them have no shell, 

 such as 



Onchidil'm, Biichau*^ 



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

 at all points, and even covering the head when it contracts. It has 

 two long retractile tentacula, and on the mouth an emarginated veil, 

 formed of two triangular and depressed lobes. 



The anus and respiratory oi-ifice are imder 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 gene- 

 ration; that of the male, on the contrary, is under the right great 

 tentaculum, the two openings being united by a furrow, which extends 

 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 airf . 



The acquatic Pulmonea, with complete shells, were also placed by 

 Linnaeus in his genera Helix, Bulla and Valuta, from which it has 

 been found necessaiy 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.+ 



The Planorbes had already been distinguished from the Helices by 

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



* Onphidium, a name given to this genus, because the first species {Onckidium 

 ti/pfue, Bnchna., Lin. Soc. Lond., V, 132) was tuberculous; I now kuow one that 

 is stnooth, the Gnchidium Icetigatum, Cuv., and four or fis'e that are tuberculous: 

 Oncli. Peronii, Cuv., Ann. du Miis., V, 6; — Onch. Sloanii, Cuv., Sloaue, Jam., pi. 

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

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



N. B. M. de Blainville has changed the name of Oncliidium into that of Peronia, 

 and applied the former to the Vaginulse. These Peronia he places among his 

 Cyclobraxchiata, but I can see no real differeace between their respiratory 

 organ aud that of the other Pulmoneae. 



t See Chamisso, Nov. Act. Mat. Cuv., XI, part I, p. 348, and Van Hassel, 

 BuUet. Uuivers., 1S24. Sept., Zool., 83. 



X Hel. vortex; — H. cornea; — H.spirorbis; — H.polygyra; — //. contorta; — H. 

 initida ; — H. alM ; — ,'/. similis. 



See the quotations of Gmel., and add, Draparnaud, pi. I, f. 39 — 5), and pi. ii, 

 f. 1—22. 



