254 MARTtN F. WOODWARD. 



richii so closely resembles P. Quoyaua in its radula, it 

 differs equally with tlie latter species from P. Adanso- 

 nianaj the radula of which, according to Dall, shows a 

 rhachian, 15 laterals, 5 tufted uncini, 4 denticulate uncini, 

 and 45 simple uncini, or 69 teeth on either side of the 

 rhachian. An examination of his figure would lead us to 

 interpret the teeth somewhat differently, but since the num- 

 bering of the teeth in the plate and account given in the text 

 are at variance a detailed comparison becomes difficult. It 

 is obvious, however, that this form differs considerably in its 

 radula from the section Ferotrochus, and thus justifies 

 Fischer's creation of the section Eutemnotrochus. 



A comparison of the Pleurotomarian radula with that of 

 other Diotocardia is almost impossible, for while the former 

 is a typical rhipidoglossate radula, yet it is so peculiar that 

 we can find no other living form which at all approximates 

 to it. This is, perhaps, not so surprising when we consider 

 the great antiquity of this form, on which account we might 

 expect that Pleurotomaria would show either a very 

 primitive type, or if, on the other hand, the radula had 

 undergone much change, that it would show a very spe- 

 cialised one. 



When we attempt to decide the question as to the primi- 

 tive or specialised nature of this radula, we are at once at 

 fault, for we have not one particle of evidence to show us 

 what the nature of the pro-rhipidoglossate radula was. All 

 the evidence we possess tells us that the Diotocardia are un- 

 doubtedly the most primitive of living Prosobranchia, and 

 that they all possess the highly developed rhipidoglossate 

 type of radula. Of the early Diotocardia, Pleurotomaria 

 is the only form of which we have any knowledge, all the 

 other living zygobranchiate Diotocardia being comparatively 

 modern forms, and this genus also shows us a rhipidoglos- 

 sate radula. It is true that the radula of Pleurotomaria 

 differs from that of all other Diotocardia in the absence of 

 those sharply marked regions which are so characteristic of 

 the majority of the i-hipidoglossate radulas. The question 



