RADULA OF THE DOCOGLOSSA 



227 



equal 6ize ; in Fissurella we again meet with a large capituliform 

 tooth, with very small laterals. 



(^) The Docoglossa are in direct contrast with the Rhipido- 

 glossa in possessing few and strong teeth, instead of many and 

 weak. There are only three families, Acmaeidae, Patellidae, 

 and Lepetidae. In some of the Acmae- 

 idae there are not more than two 

 teeth in a row, while in no species are 

 there more than twelve. The radula 

 is, however, very long; there are as 

 many as 180 rows in Patella vulgata. 

 The teeth are thick, generally of a 

 very deep red horn colour, rather 

 opaque. The cartilage in which they 

 are set is remarkably thick, and in 

 some species whose teeth are very few Fig.isi.— Portion of the raduia 



., ,, ,. c ,1 . ,-1 of Patella cretacea Reeve, 



a considerable portion of this cartilage seen in half profile, x 40. 

 is left quite bare. 



Although the teeth are so few, the arrangement is by no 

 means simple. The special feature of the group is the multipli- 

 cation of identical centrals. Of these there are two in Acmaea^ 

 and four, as a rule, in Patella. Thus in these two genera there is 

 seldom an absolutely central tooth. Either laterals or marginals 

 are liable to be lost, but there are never more than two of either 

 in Acmaea, and never more than two laterals and three marginals 

 in Patella. Thus the formula varies from 0.0.(1 + + 1).0.0 in 



Fig. 132. — Two rows of 

 the radula of PterO' 

 trachea mutica Les., 

 Naples. X 60. 



Pectinodonta, 2.2.(1 + -h 1). 2.2 in Collisellina (both Acmaei- 

 dae), to 3.2.(1 -f + 1).2.3 in Patinella, and 3.1.(2 + -}- 2).1.3 

 in Patella proper. In the Lepetidae there is an absolutely 

 central tooth, which appears to be made up of the coalescence 

 of several teeth, no laterals, and about two marginals ; formula, 

 2.0.1.0.2. 



