THE ANATOMY OF PLEUEOTOMAKIA BEYRIOHII. 253 



braue. These accessory plates, of which there are about 

 thirty-seven on each side of the middle line iu each row of 

 teeth, are situated about in the middle of each half-row of 

 teeth, commencing with the twenty-seventh and extending 

 out as far as about the sixty-fourth tooth. They appear to 

 alternate with the true bases of the teeth in front, whereas 

 posteriorly they underlie them. The row represented in fig. 

 32 would underlie the next posterior row of teeth. 



Comparison with the Radula of P. Quoyana and 

 P. Adansoniana. 



The radulae of the three species of Pleurotomaria, of 

 which the animals have been examined, stand apart from 

 those of all other Diotocardia iu the absence of that sharp 

 division into regions which is so characteristic of the ma- 

 jority of this group. They are further to be distinguished 

 by the character of their central teeth, and in the possession 

 of brush teeth. 



Of the two species, P. Quoyana much more nearly ap- 

 proaches P. Beyrichii iu the character of its radula than 

 does P. Adansoniana. The radula of the former, accord- 

 ing to Bouvier and Fischer, may be expressed as follows : — 

 El. 3, 24, 13, 63, 6, there being 109 teeth on either side of the 

 rhachian. Except iu the number of the teeth in the different 

 groups there is very little difference indeed between the two 

 species, the resemblance being so close that one might almost 

 match the individual teeth in the two radulas ; thus the rhachi- 

 dian, the central pairs, and the lamellate are very similar, 

 the only difference being in the greater number of lamellate 

 teeth (twenty-four) in P. Quoyana. The thirtieth tooth of 

 the latter species forms an exact match with the twenty- 

 sixth of P. Beyrichii, and the fiftieth with the forty-third. 



This close resemblance between the radulge of these two 

 species is strong argument iu favour of the retention of 

 these two forms in a sub-section of the genus Pleuroto- 

 maria (section Perotrochus, Fischer). Since P. Bey- 



