Io THE LIFE OF THE MOLLUSCA 
The mandible is single, and placed on the upper 
side of the mouth in the Limpets (Patellidz), one of 
the Sea-Slugs (42gives punctilucens), the Land and 
Freshwater Snails (Pulmonata, Plate III., Figs. 5 
and 6), and the Elephant’s-Tusk Shells (Dentaliidz). 
Two kinds of Pond Snails (Limnaa and Planorbis) 
have three mandibles, an upper and two side plates 
(Plate III., Fig. 7). The majority of the other 
gastropods—for the carnivorous species are generally 
without them—have two lateral plates. Many of 
these are ornamented with most elaborate patterns, 
and some have raised projections on them like a file 
(Plate III., Fig. 8). 
The Cephalopoda also have a pair of mandibles, 
upper and lower, resembling a Parrot’s beak, save 
that the upper fits within the lower one (Plate III., 
Fig. 4). 
These mandibles are formed apparently of dense 
chitine, strengthened in the case of Chatoderma and 
Nautilus with carbonate of lime. 
The radula is so important a feature as to call for 
more detailed mention.. With its supporting car- 
tilages and muscles it occupies the position in the 
mouth assumed by the tongue in the higher animals, 
and much resembles that organ in general appearance. 
The radula itself, however, consists of a series of 
recurved teeth formed of dense chitine, attached in 
transverse rows to a membrane of the same sub- 
stance. In the Limpets (Docoglossa) the teeth 
