228 



RADULA OF HETEROPODA AND AMPHINEURA 



CHAP. 



The radala of the Heteropoda is quite characteristic, and 

 shows no sign of affinity with any other Prosobranchiate. The 

 central tooth is large, broad, tricuspid, and denticulated on a 

 broad base ; the single lateral is strong, often bicuspid ; the two 

 marginals simple, long, falciform; formula, 2.1.1.1.2 (Fig. 132). 



Amphineura. — (rt) Polyplacoplwra. — The radula of the 

 Chitonidae is quite unique. It resembles tliat of the Docof/Iossa 

 in being very long, and composed of thick and dark horn-col- 

 oured teeth. The number of teeth, however, is considerably 



Fig. 1o3. — A, Portion of the radula of Chiton {Acanthopleura) spiniger Sowb., An- 

 damaus, x 30 ; B, portion of the radula of Dentalium entails L., Clyde, x 50. 



greater, amounting almost invariabl}^ to seventeen in each row. 

 There are three rather small central teeth, the two outer of 

 these being similar ; next comes a very large lateral (the major 

 lateral), usually tricuspid, which is followed by two much 

 smaller laterals, which are scarcely more than accessory plates ; 

 then a very large and arched marginal (the major uncinus), at 

 the outer side of which are three accessory plates. Some con- 

 sider there is only one central tooth, and count the two small 

 teeth on each side of it as laterals. 



Thus the formula is either (3 + 1) . (2 + 1) .3. (1 + 2) . (1 + 3) 

 or (3 + l).(2 + l+l).l.(l + l + 2).(l + 3). 



(^) Aplacopliora. — Of this rather obscure order, Chaetoderma 

 has a single strong central tooth, Neomenia has no radula. 



