MALACOZOA. GASTEROPODA. Pl'LMOBRANCHIATA. 113 



the margins extended into digitiform contractile lobes, which 

 can be spread out on the columellar side, behind, and over 

 part of the outer lip of the shell, and may sometimes be seen 

 meeting above ; the veil, or supraoral flap broad, emarginate, 

 projecting over the anterior edge of the foot. 



In its ordinary mode of progression under the water, it glides 

 along with a uniform, moderately quick motion performed by 

 minute undulations of the foot, which does not then extend to 

 the anterior edge of the shell, but projects a little beyond its 

 hind part or apex ; the margin of the head or flap scarcely 

 appearing, the tentacula extended. It moves equally well in 

 a reversed position, with the foot applied to the surface of the 

 water. Ordinarily, the digitations of the mantle are not ex- 

 tended. If the animal be turned on its back in the water, it 

 protrudes the foot, head, and tentacula, writhes the foot, of 

 which the margins then form obtuse lobes, and emits the digi- 

 tations of the mantle which move in various directions, until it 

 either ascends to the surface or regains its ordinary position, 

 when the mantle-lobes are withdrawn. When in shallow water, 

 beyond the surface of which part of the shell protrudes, it ex- 

 tends the head and neck, as well as the foot, the latter then 

 not reaching as far as the end of the spire, and at intervals 

 jerks forwards the shell. In the air, it does not erect the 

 tentacula, and advances by jerks. Its movements are rather 

 quick, and it turns with rapidity on meeting an obstacle. 

 Montagu states that it " will sometimes let itself down gradu- 

 ally by a thread affixed to the surface of the water, in the 

 manner of the Limax filans from the branch of a tree ;" but I 

 have not seen it do so. 



Planorbis Bulla. Muller, Venn. Terr, et Flu v. ii. 167. — Phvsa 

 fontinalis. Drap. Moll. Terr, et Fluv. 54. PI. 3. f. 8, 9.— Bulla 

 fontinalis. Linn. Syst. Nat. 1 185. — Bulla fontinalis. Mont. Test. 

 226.— Phvsa fontinalis. Lamk. Syst. vi. 2. 156; Ed. n. viii. 40 L 

 — Phvsa fontinalis. Flem. Brit. Anim. 276. — Buccinum exiguum, 

 trium spirarum a sinistra in dextram convolutarum. Lister. Anim. 

 Angl. 142. PI. 2. f. 25. 



Genus 3. Planorbis. Coil-Shell. 



Animal very slender, elongated, involute ; head with 

 two very long, tapering, setaceous tentacula, bearing the 

 eyes on their dilated base internally ; mouth surmounted 

 by a bilobate veil, and having a crescent-shaped tooth 

 above, and a lingual mass armed with small hooks below ; 



k2 



