VIII 



FORMULAE OF TEETH 



219 



both their central and lateral teeth, and that those which remain 

 are true uncini or marginals. Spirotropis appears to be the soli- 

 tary survival of a group retaining the primitive form of radula. 

 The arrangement of teeth in all these sections is expressed 

 by 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 Phil., Norway, x 70. 



Fig. 113. — Rudwlaof Bel at wricula Mont. Fig. 115. — Eight teeth from the radula 

 X 70. of 7'e;*e6n< caeru/escens 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, wliich has also one lateral on 

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



the formula is c^iven thus: ' ' ' ' , ' ' ' ' , where 30 and 42 

 ^ 30 42 



stand for the average number of ro2vs of teeth in Co7ius and 



Spirotropis respectively ; the same is sometimes expressed thus : 



1.0.0.0.1 X 30; 1.1.1.1.1 X 42. 



