182 HELICID.E. 



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 

 were 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 y Pijrula 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. Mutter, 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. 57.; Brard, p. 83. t. 3. f. 9.; Alder, Mag. Zool. 

 and Bot. ii. 110.; Pfeiffer, 60. t. 3. f. 25.; Rossm. Icon. i. 

 76. t. 2. f. 29. Clausilia lamellata. Leach, Moll. Syn. 85. 

 Helix (Cochlodina) derugata. Per. Tab. 63. Clausilia 

 laminata. Turton, Man. ed. 1. 70. f. 53. ; Moquin Tand. 



