XI] OF VARIOUS UNIVALVES 819 



save in the somewhat "produced" spire, that is to say in the com- 

 paratively low value of the angle p. 



A variation with advancing age of ^ is common, but (as Blake 

 points out) it is often not to be distinguished or disentangled from 

 an alteration of a. Whether alone, or combined with a cKange in a, 

 we find it in all those many gastropods whose whorls cannot all be 

 touched by the same enveloping cone, and whose spire is accordingly 

 described as concave or convex. The former condition, as we have 



Fig. 398. Trochus niloticus L. 



it in Cerithium, and in the cusp-Hke spire of Cassis, Dolium and 

 some Cones, is much the commoner of the two*. 



In the vast majority of spiral univalves the shell winds to the 

 right, or turns clockwise, as we look along it in the direction in which 

 the animal crawls and puts out its head. The thread of a carpenter's 

 screw (except in China) runs the same way, and we call it a "right- 

 handed screw." Save that it takes a right-handed movement to 



* Many measurements of the linear dimensions of univalve shells have bgen 

 made of late years, and studied by statistical methods in order to detect local races 

 and other instances of variation and variability. But conchological statisticians 

 seem to be content with some arbitrary linear ratio as a measure of "squatness" 

 or the reverse; and the measurements chosen give Uttle or no help towards the 

 determination either of the apical or of the spiral angle. Cf, (e.g.) A. E. Boycott, 

 Conchometry, Proc. Malacol. Soc. xvn, p. 8, 1928; C. Price- Jones, ibid, xix, 

 p. 146, 1930; etc. See also G. Duncker, Methode der Variations-Statistik, Arch, 

 f. Entw. Mech. vm, pp. 112-183, 1899. 



