THE ANATOMY OF PLEUROTOMAEIA BETRICHII. 255 



then arises as to whether, taking into consideration the anti- 

 quity of Pleurotomaria, we are justified in regarding this 

 feature as a primitive one. 



Considerable stress has been laid by Troschel, Moore, and 

 others upon the breaking up of the rhipidoglossate radula 

 into zones, three on either side of the rhachiau tooth, and on 

 the occasional replacement of the great group of marginal 

 laterals by one large tooth, which, however, generally re- 

 tains sufBcient traces of the individuals which it replaces 

 to suggest that it represents a fusion of teeth, a view which 

 is supported when we find that this takes place in undoubtedly 

 specialised forms (notably in Addisonia and Cocculina 

 among the Rhipidoglossa, and certain Cyclophoridae among 

 the archi-Taenioglossa). Such a condition has led some to 

 suppose that the tooth arrangement met with in the Taftnio- 

 glossa might be derived from the Rhipidoglossate radula 

 by a fusion of the elements of the three zones, thus giving a 

 formula of 1.1.1.1.1.1.1, a view which the condition of the 

 archi-Taenioglossate Cyclophoridge seems to support. 



If, then, this subdivision of tbe row of teeth into sharply 

 marked zones is a foreshadowing or a tendency in the direc- 

 tion of the condition met with in the Tsenioglossa, it seems 

 only natural to conclude that this in turn was derived from 

 a radula in which all the teeth in a transverse row were 

 similar. Such a stage has not been preserved to us, but in 

 Pleurotomaria we have an approximation to this condition, 

 inasmuch as all the various specialised tooth areas merge 

 imperceptibly into one another, and this in my opinion is a 

 very primitive character. 



I therefore conclude that, in spite of its very specialised 

 brush teeth, the radula of Pleurotomaria exhibits the most 

 primitive type among all existing Gastropods. 



Considerations regarding the Primitive Nature 

 of Pleurotomaria. — If we are justified in concluding, as I 

 have done above, that in its radula Pleurotomaria is a 

 most primitive form, then we might naturally expect to find 

 indications of this primitive character in other of its organs. 



