212 PNEUMONOBRANCHIATA. 



may, however, at any time be relaxed by a little 

 moisture, when it will instantly resume its elasticity, 

 and spring from its attachment." 



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 monstrosites, but now it is well known 

 that some species which are generally dextral, are 

 often found reversed; and this monstrosity consists 

 not only in the shell being turned in the contrary 

 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 Bu- 

 limics 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 like other shells. (See Phil. Trans. 1833.) 



* Marpessa Gray. 



Clausium notched at the tip, fitting into a plait on 

 the outer lip of the shell; shell smooth. 



78. 1. Clausilia bidens. Laminated Close Shell. 



(t. 5. f. 53.) Shell nearly smooth, glossy, and 



transparent; aperture with two white plaits 



clausium emarginate. 

 Helix bidens. Milller, Verm. ii. 116.; not Linn. 

 Turbo laminatus. Mont. p. 359. t. 11. f. 4. 

 Clausilia bidens. Drap. p. 60. t. 4. f. 5-7. ; Brard, 



p. 83. t. 3. f. 9. ; Alder, Mag. Zool. Sf Bot. ii. 



