416 DR. A. H. COOKE ON THE 



Group (8) of variegata. 



RhaGhidian single-cusped on a triangular plate : laterals de- 

 graded, with one lai^ge cusp only. 



M. variegata Reeve. Mauritius. This unique and most in- 

 teresting form shows a rhachidian formed of a simple triangular 

 plate, with 2 angles at the extremes of the upper margin, and the 

 3rd in the centre of the base ; in the middle of the upper margin 

 is a single sharp, deeply rooted cusp, the tip of which projects well 

 beyond the margin, no trace of any other cusp : laterals in general 

 shape like interlirata, with a single strong denticle of exaggerated 

 size, inclining inwards, no trace of any further denticles ; the 

 projecting " keel " much larger than in interlirata. 



Text-figure 14. 



Lateral and rliacliidiaii tootli of Mitra variegata Reeve. 



'~ The whole radula is rather thin and lacking in substance; 

 while the merging of all the cusps in the laterals in a single 

 strong denticle is very suggestive of the lines on which radula 

 development may proceed. 



Group (9) of cermnnosa. 



Radula small, rhachidian and laterals set with numerous small 

 sharp cusps ; framework of teeth narrow, nearly straight. 



1. M. cerumnosa Melv. S. Africa. Rhachidian 8-cusped, the 

 6 interior equal in length, the 2 exterior much smaller ; cusps 

 sharp, small, none deeply rooted ; base slightly arched : laterals 

 with 12-14 cusps, the 2 or 3 interior inclining slightly, but all in 

 the upper margin gradually diminishing in size. 



2. M. hovei Kien. Suez. Rhachidian 8-9-cusped, cusps nearly 

 equal, not deeply rooted ; base very slightly arched: laterals with 

 12 nearly equal cusps, with 2 or 3 serrations at the outer end. 



Text-ficfure 15. 



Lateral and rliacliidian tooth of Mitra hovei Kien. 



3, M.filosa Born. Mauritius. ii'Aac/^icZiaH 8-10-cusped, cusps 

 very small, framework narrow, upper margin slightly curved ; the 

 €usps are inclined to become irregular in different teeth, and 



