i8o HELICIDJE. 



The number of the folds and teeth with which the 

 mouth of the shell of C. tridens (as well as of some 

 species of other genera) is furnished is not constant. 



Var. crystallina, Dupuy. Shell greenish-white, transparent, 

 and glassy. Wheeley Castle, Worcestershire (Clark), near 

 Stanstead, Kent (Smith), Tawstock Woods, near Barnstaple, and 

 Brockley Combe, Somersetshire (J. G. J.), B.C. Petersfield, 

 Hants (C. Ashford), near Birmingham (G. Sherriff Tye), near 

 Wakefield (Jackson)/^ G. Taylor, J.C. 



Monst. sinistrorsa. Shell reversed. A specimen found by 

 Mr. J. Emmet, of Boston Spa, J.C., July, 1879. 



In the ' Journal of Conchology ' for July, 1879, there 

 is a note on C. tridens by Mr. William Taylor, who 

 proposes to add to the British list two varieties of 

 that species, viz. C. Noulitiana (Dupuy) and C. 

 Alzensis (St. Simon), of which he gives the following 

 descriptions : 



" Var. Noulitiana. Shell rather larger and thinner, a single 

 denticle only on the outer lip." 



Mr. Taylor considers this to be " our common 

 English form." 



Var. Alzensis. " Outer lip bearing two deeply-seated denticles 

 in addition to those described in the typical form ; found at 

 Dorridge, &c., Warwickshire." 



The inconstancy of the number of folds or teeth in 

 the aperture of nearly every species in the genera 

 Pupa, Vertigo, and Clausilia is remarkable ; and if 

 the presence or absence of a tooth or two in C. tridens 

 is held to be a sufficient reason for making varieties, 

 the same will hold good in their case also ; but the 

 needless multiplication of varieties, as well as species, 

 ought, I think, to be avoided. 



