264 MEBISTIC VARIATION. [part i. 



outside of the central plate a pair of almost wholly separate 

 denticles on one side correspond with a large, imperfectly divided 

 denticle of the other side. A very similar specimen is figured by 

 Friele, Norske Nordhavs-Exp., viii. PL v. fig. 16. 



The number found in one part of the radula is usually main- 

 tained throughout the whole series, but this is not always so. 

 A case in which the number of centrals at the anterior end of 

 the radula was 6, and at the posterior end 8, is given by Friele, 

 Norske Nordhavs-Exp., 1882, viii. p. 27, Taf. v. fig. 17. 



