228 



RADULA OF IIETEROrODA AND AMPHINEURA 



The radula, of the HcUrojiioda is quite characteristic, and 

 sliows no sign of aiHuity with any other Prosohranchiate. The 

 central tooth is large, l)road, tricuspid, and denticulated on a 

 broad base ; tlie single lateral is strong, often Ijicuspid ; the 

 two marginals simple, long, falciform; formula, 2.1.1.1.2 

 (Pig. 132). 



Amphineura. — (cO Polyplacophora. — The radula of the 

 Chitonidae is quite mucpie. It resembles tliat of the Docof/lossa 

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

 coloured teeth. The nund)er of teeth, however, is considerably 



rJG, 133. — A, Portion of the radula of t'liUon [Acanthopleura) spiniger Sowb., Anda- 

 raans, x 30 ; B, portion of tlic radula of Dentalium entalis L., Clyde, x 50. 



greater, amounting almost invarial)ly to seventeen in eacli row. 

 There ar(^ three rather small central teeth, tlie two outer of these 

 being sindlar ; next comes a A^'ery large lateral (the major lateral), 

 usually tricuspid, which is fulltnved 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 

 winch are three accessory plates. Some consider there is only 

 one central tooth, and count tlie two small teeth on eacli side of 

 it as laterals. 



Thus the fornmla, is citlier (.'5 + 1) - (2 + 1) .3. (1 + 2) . (1 + 3) 

 or (3 + 1) . (2 + 1 + 1) .1. (1 + 1 + 2) . (1 + 3). 



(^) Aplauyphora. — Of tins rather obscure order, Cliactodcrma 

 lias a single strong central tooth, Keomenia has no radida. 



