406 



DR. A. H. COOKE ON THE 



cuGumer^na 



digitalis 



ei^iscojycdis 



ferruginea 



fidva 



glabra 



papalis 



In many genera of rhacliiglossate Mollusca, and particularly in 

 the Thaidie and allied groups, the form of the laterals is neai'ly 

 constant, cind affords little help in subdivision and classification ; 

 the shape of the rhachidian is the determining factor. In the 

 Mitrida3 the reverse is the case, for while the rhachidian tooth 

 varies considerably in shape, the modifications of the laterals are 

 of great importance, and nppear to be of equal or even superior 

 value in estimating the relationships of allied groups and species. 

 Thus, VexiUum is at once separated from all other forms of Mitra 

 by its single-hooked laterals. 



Similar, though not so wide divergencies in the laterals form a 

 basis for classification within the various groups. In this respect, 

 the Mitridfe fall into line with the allied genera Fusinus^ Fascio- 

 laria, Latirus, and Feristernia, in all of which the laterals, rather 

 than the rhachidian tooth, supply the best evidence for classifica- 

 tion. In these four genera, the rhachidian is so much reduced 

 in size that it has practically lost those characteristics which can 

 be employed to distinguish one group fiom another. The 

 Mitridse have not quite reached tliis point of development, or, as. 

 it might be called, of degradation, but the rhachidian is small in. 

 proportion to the rest of the lingual apparatus. 



Group 1. Mitra Marty n. 



Fhachidian Q-1 1 'Cusped on a squarish "or oblong framework: 

 laterals long rake-shaped, multicuspid, the interior cusps inclining 

 slightly inward, cusps 2, 3, or 4 slightly the largest. 



Text-figure I. 



Lateral and rliacliidian tcotli oi IlUi-a adiista Lam. 



Section (a). il/?fo'« proper. — Fhachidiaoi with 8-11 deeply rooted 

 sharp cusps, the outermost of which lie obscurely behind the 

 others. 



1. JI. adusta Lam. Roturna. Rhachidian lO-cusped, the 6 

 inner cusps nearly equal, the 2 outer on each side smaller, and lying 

 somewhat behind and below the others; base nearly straight^ 



