RADULA OF THE PTENOGLOSSA, ETC. 



225 



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

 tooth, if present (it does not occur in lanthinci), is the smallest 

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

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

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

 the middle. 



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

 tooth in Scalaria is or is not reckoned to exist. 



(e) Gymnoglossa. — In the absence of both jaw and radida it is 

 not easy to classify the two families (Eulimidae and Pyrami- 

 dellidae) which are grouped under this section. Fischer regards 

 them as modified Ptenoglossa ; one would think it more natural 

 to approximate them to the Taenioglossa. 



Fifi. 129. — Portion of the radula of Margarita umhilicalis Brod., Labrador. 

 X 75 and 300. 



(/) Uliipidoglossa.— This section consists of seventeen families, 

 the most important being the Helicinidae, Neritidae, Turbinidae, 

 Trochidae, Haliotidae, Pleurotomariidae, and Fissurellidae. The 

 radula is characterised hj — 



(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, ])ut curving 

 away backwards as they become smaller ; in Solaridla alone, 

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



(2) The varying nundjer of the laterals. The average 



VOL. Ill Q 



