226 



RADULA OF THE RHIPIDOCxLOSSA 



numl^er of these is live on eacli side ; in some cases {Livona^ 

 there are as many as nine, in some (JVeritojJsis) 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 nncinus, 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, Livoiia 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 

 rhipidoglos-sate formula, the first being that to which preference 

 is given, viz. : — 



(1) 00.5.1.5.00 , i.e. one central, five laterals, including 

 the ' last lateral ' tooth. 



(2) ( CO. 1). 4.1.4. (1.00 ), regarding the 'last lateral" as first 



uncinus, but specialising it by a number. 



(3) cc. 1.(4. 1.4). l.co , regarding the ' last lateral ' as the only 

 lateral. 



In the Neritidae and the derived fresh-water genera {Neritina, 

 Navicella) the first lateral, as w^ell as the capituliform tooth, is 



Fig. ]30. —Portion of the radiila oi Nerita albicillu, L. , Andaman Is., with ctntral tooth 

 highly magnitied : c, c, the capitnlif'orm tooth. x 40. 



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

 of mutton ; the intervening laterals are very small. In 

 Neritopsis (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. Hydrocena is a very 

 aberrant and apparently degraded form; the laterals between the 

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

 Scissurella, and Fleurotomnria the five laterals are of fairly equal 



