236 



KADULA OF SCAPHOPODA AND CEPHALOPODA 



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 tri- 

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

 Phijsa, laterals comb-like, serrations not deep. 



Am2Jhibolidae : Central tooth five-cusped on a l)road Imse, 

 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 (JDentcdiidae) the radula is 

 large, and (piite 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; fornnda, 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 conqjared 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 Eledone and 

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

 unicuspid, in Tremoctoims tricuspid. The laterals are always 



tliree in iiumber, the in- 

 nermost lateral having a 

 tendency t(3 assume the 

 form of the central. In 

 Sepia the two inner laterals 

 are exact reproductions of 

 the central tooth ; in Ele- 

 done, Sepiola, Loligo, and 

 Sepia, the third lateral is falciform and mucli the largest. 



In Nautilus, the only living representative of the Tetra- 

 branchiata, there are two sickle-shaped marginals on eacli side. 



Fig. 142. 



—Portion of tlic radula of Octopus ietra- 

 cirHuis D. Ch., Naples, x 20. 



