FORMULAE OF TEETH 



219 



and lateral teeth, and that those which remain are true uncini 

 or marginals. Spirotropis appears to be the solitary sm-vival of 

 a group retaining the primitive form of radula. 



The arrangement of teeth in all these sections is expressed 

 liy a formula applicable to each transverse row of the series. 

 The central tooth, if present, is represented by 1, and the laterals 

 and marginals, according to their number, on each side of the 



Fig. 114. — Portion of radula of Spiro- 

 tropis carinata Pliil., Norway. x 70. 



Fig. 113.- 



-Radula of Bela turricula Mont. 

 x70. 



Fig. 115. — Eight teetli from the radula 

 of Terebra caerulescens Lam. x 60. 



central figure. Thus the typical formula of the Toxoglossa is 

 1.0.0.0.1, the middle standing for the central tooth which is 

 absent, and the on each side of it for the absent laterals ; the 

 1 on each extreme represents the one uncinus in each row. 

 Thus the formula for Spirotropis, which has also one lateral on 

 each side and a rachidian or central tooth, is 1.1.1.1.1. Often 



1.0.0.0.1 1.1.1.1.1 



the formula is u'iven thus : - 



where 30 and 42 



10 42 



stand for the average number of roivs of teeth in Conus and 

 Spirotroins respectively ; the same is sometimes expressed thus : 

 1.0.0.0.1 X 30: l.l.l.i.l x42. 



