THE ANATOMY OF PLEUROTOMARIA BEYRICHII. 247 



in PI euro torn aria. Even in Troclius I think they greatly 

 overrate the size and importance of this nerve, which so far 

 as I can find is a small nerve distributed to the posterior 

 portion of the mantle, and probably to the secretory epithe- 

 lium of that region. 



A few small nerves arise from the cerebro-pedal commis- 

 sui-e, these being distributed to the muscles between the 

 under lip and the foot. 



The Sense-organs. — The eyes and osphradia having 

 been already described, it only remains to draw attention to 

 the oto cysts. These latter take the form of a pair of large 

 vesicles, situated just above and in front of the pleuro- 

 pedal cords (fig. 27). The actual otocyst is not very large, 

 but it is surrounded by a very tough, thick, concentric ar- 

 ranged sheath of connective tissue (fig. 31 -4). The otoconia 

 are small and numerous ; typically they are spherical bodies, 

 varying much in size and often fusing together to form 

 reniform structures (fig. 31 B). 



The Radula (figs. 32— 52).— The radula of Pleuroto 

 maria Beyrichii is extremely complex, and exhibits the 

 same type as that described for P. Adansoniana by Dall, 

 and P. Quoyana by Fischer and Bouvier. In the number 

 and character of its teeth it more closely approximates to the 

 latter species — a fact which strongly supports the view ad- 

 vanced by Crosse in 1882, from the study of the shells, that 

 these two species should be grouped together as a section or 

 sub-genus of Pleurotomaria, for which group P. Fischer 

 proposed the name Perotrochns. 



The radula is very large, one example measuring 62 ram. 

 long by 5 mm. wide. The greater portion of the radula is of 

 course not functional, but lies buried in the radular sac, 

 which extends up to the anterior lobe of the right kidney. 



In fig. 32 I have given a view of half a transverse row, 

 which, as mentioned by Fischer and Bouvier, does not run 

 straight across the radula, but has a somewhat V-shaped 

 course. 



The number of teeth in a transverse row is 223; one of 



