RADULA OF THE PTENOGLOSSA, ETC. 



225 



teeth, of which the outermost are the largest. The central 

 tooth, if present (it does not occur in lanthina^, is the smallest 

 in the series, and thus recalls the arrangement in some of the 

 carnivorous Pulmonata (p. 232). In lanthina the radula is 

 formed of two large divisions, wdth a gap between them down 

 the middle. 



The formula is oo.l.oo or go.O.go according as the central 

 tooth in Scalaria is or is not reckoned to exist. 



(e) G-ymnoglossa. — In the absence of both jaw and radula 

 it is not easy to classify the two families (Eulimidae and Pyra- 

 midellidae) which are grouped under this section. Fischer 

 regards them as modified Ptenoglossa ; one would think it more 

 natural to approximate them to the Taenioglossa. 



Fig. 129.- 



Portion of the radula of Margarita umhilicalis Brod., Labrador. 

 X 75 and 300. 



(/) BMpidoglossa. — This section consists of seventeen 

 families, the most important being the Helicinidae, Neritidae, 

 Turbinidae, Trochidae, Haliotidae, Pleurotomariidae, and Fissu- 

 rellidae. The radula is characterised by — 



(1) The extraordinary development of the uncini, of which 

 there are so many that they are always reckoned as indefinitely 

 numerous. They are long, narrow, hooked, and often cusped 

 at the top, and crowded together like the ribs of a fan, those at 

 the extreme edge not being set straight in the row, but curving 

 away backwards as they become smaller; in Solariella alone, 

 where there are from five to ten, can they be counted. 



(2) The varying number of the laterals. The average num- 



VOL. IIJ Q 



