236 RADULA OF SCAPHOPODA AND CEPHALOPODA chap. 



marginals serriform, comb-like, with a wing-like appendage at 

 the superior outer edge (Fig. 141, C). 



Chilinidae : Central tooth small, cusped on an excavated 

 triangular base, marginals five-cusped, with a projection as in 

 Physa^ laterals comb-like, serrations not deep. 



Amphiholidae : Central tooth five-cusped on a broad base, 

 central cusp very large ; two laterals only, the first very small, 

 thorn-like, the second like the central tooth, but three-cusped ; 

 laterals simple, sabre-shaped. 



Scaphopoda. — In the single family QDentaliidae) the radula 

 is large, and quite unlike that of any other group. The central 

 tooth is a simple broad plate ; the single lateral is strong, arched, 

 and slightly cusped ; the marginal a very large quadrangular 

 plate, quite simple; formula, 1.1.1.1.1 (Fig. 133, B). 



Cephalopoda. — The radula of the Cephalopoda presents no 

 special feature of interest. Perhaps the most remarkable fact 

 about it is its singular uniformity of structure throughout a large 

 number of genera. It is always very small, as compared with 

 the size of the animal, most of the work being done by the 

 powerful jaws, while the digestive powers of the stomach are 

 very considerable. 



The general type of structure is a central tooth, a very few 

 laterals, and an occasional marginal or two ; teeth of very uniform 

 size and shape throughout. In the Dibranchiata, marginals are 

 entirely absent, their place being always taken, in the Octopoda, 

 by an accessory plate of varying shape and size. This plate 

 is generally absent in the Decapoda. The central tooth is, in 

 the Octopoda, very strong and characteristic ; in JEledone and 

 Octopus it is five-cusped, central cusp strong; in Argonauta 

 unicuspid, in Tremoctopus tricuspid. The laterals are always 



three in number, the in- 

 nermost lateral having a 

 tendency to assume the 

 form of the central. In 

 Sepia the two inner laterals 

 are exact reproductions of 



Fig. 142.— Portion of the radula of Oc?op«,s^e^ra- the Central tOOth ; in Fh- 

 c/?r/ii<s D. Ch., Naples, x 20. _ rr • 7 t ?• j 



done^ jSepiola, LoUgo^ and. 



Sepia^ the third lateral is falciform and much the largest. 



In Nautilus^ the only living representative of the Tetra- 



