226 



RADULA OF THE RHIPIDOGLOSSA 



CHAP. 



ber of these is five on each side ; in some cases QLivona) there 

 are as many as nine, in some (^Neritopsis') only three. The 

 lateral next to the uncini (which is specially large in the 

 Neritidae, and is then known as the capituliform tooth) is 

 regarded by some authorities as the first uncinus, by others as 

 the sole representative of the laterals, the teeth on the inner 

 side of it being reckoned as multiplied central teeth. Accord- 

 ing to this latter view, Livona will have as many as seventeen 

 central teeth. Taking five as the average number of ' laterals,' 

 we shall have the following different ways of constituting the 

 rhipidoglossate formula, the first being that to which preference 

 is given, viz.: — 



(1) co.5.1.5.00 , i.e. one central, five laterals, including the 

 ' last lateral ' tooth. 



(2) (oo.l).4.1.4.(l.oo ), regarding the 'last lateral' as first 

 uncinus, but specialising it by a number. 



(3) oo.l.(4.1.4).l.oo , regarding the 4ast lateral ' as the only 

 lateral. 



In the Neritidae and the derived fresh-water genera (^JVeritina, 

 Navicella) the first lateral, as well as the capituliform tooth, is 



Fig. 130. — Portion of the radula of Nerita alhicilla L., Andaman Is., with central 

 tooth highly magnitied : c, c, the capituliform tooth, x 40. 



very large, and in shape rather like the blade bone of a shoulder 

 of mutton ; the intervening laterals are very small. In Neri- 

 topsis (a degraded form) the central tooth and first lateral 

 are entirely wanting. In the neritiform land-shells {Helicina,. 

 Proserpina) the first lateral is no larger than the others,, while 

 the capituliform tooth is enormous. Hydrocencu is a very aber- 

 rant and apparently degraded form; the laterals between the^ 

 first and the capituliform tooth are all wanting. In Haliotis,. 

 ScisMireUa,, and Fleyj-o.tomaria the five laterals ar^ of fairly 



