182 
HELICID^. 
Montagu, who described the animals of all the 
species he knew, enters into a long explanation re- 
specting the difference between sinistral, or reversed, 
and dextral shells. Formerly, all reversed shells 
Avere considered as monstrosities ; but now it is well 
known that some species which are generally dextral, 
are often found reversed ; and this monstrosity con- 
sists not only in the shell being turned in the con- 
trary direction down the imaginary axis, but the 
animal itself has all the organs placed on the opposite 
side of the body. There are some few Mollusca 
which appear to be very liable to this monstrosity, 
as Bulimus aureus^ Pyrula perversa^ and the whole 
genera of Clausilia and Physa^ the natural character 
of which is to be reversed. It would be a monstrosity 
in them to find them dextral or twisted in the same 
direction as other shells. (See Phil. Trans. 1833.) 
a. Clausium notched at the tip ^fitting into a plait on 
the outer lip of the shell ; shell smooth. (Mar- 
pessa Gray.) 
83. 1. Clausilia bidens. Laminated Close Shell. 
— Shell nearly smooth, glossy, and transpa- 
rent ; aperture with two white plaits ; clausium 
emarginate. (t. 5. f. 53.) 
Helix bidens. Muller^ Verm.ii. 116. {not Linn.'). — Bulimus 
bidens. Brug. E. M. 352. - — Turbo laminatus. Mont. 
p. 359. t. 11. f. 4. — Clausilia bidens. Drap. p. 68. t. 4. 
f. 5—7. ; Brard, p. 83. t. 3. f. 9. ; Alder, Mag. Zool. 
and Bot. ii. 110.; Pfeiffer.^ 60. t. 3. f. 25.; Bossm. Icon. i. 
76. t. 2. f. 29. — Clausilia lamellata. Leach., Moll. Syn, 85. 
— Helix (Cocblodina) derugata. Fer. Tab. 63. — Clausilia 
laminata. Turton^ Man. ed. 1. 70. f. 53. ; Moquin Tand. 
