BADULA OF THE MITRIDA^. 409 



The complete absence of any small cusps on the side of the 

 rhachidian, which often makes it dilRcnlt to count their actual 

 number, seems to justify the creation of a separate section for 

 this species. 



Group 2. Subgenus Dibaphus Philippi. 



Rhachidian closely allied to that of Mitra proper, 8~9-cusped : 

 laterals multicuspid as in Mitra. 



D. edentulus Reeve. Polynesia. Rhachidian with 8-9 deeply 

 rooted sharpish cusps, projecting well beyond the upper margin, 

 the 5 central the longest ; upper margin slightl}'' curved, base 

 slightly arched : laterals with 19-21 deeply rooted sharp cusps, 

 set well apart, gradually diminishing to the outer end of the 

 framework. 



Text-figure 4. 



Lateral and rhachidian tooth of Dibaphus edentulus Reeve. 



A specimen from Mauritius has 9 cusps on the rhachidian 

 and 18-19 on the laterals, another (Cambridge) from Mauritius 

 has 43 + nascent rows. 



Group (3) of coriacea. 



M. coriacea Reeve. S. Pacific. Rhachidian 3-cusped, cusps 

 thin and sharp, the central 3 times as long as the others, all 

 projecting well beyond the upper margin, which is much curved, 

 sloping sharply away from the central cusp ; on the outer sides of 

 the central mass are two lateral projections, like broad denticles; 

 base slightly curved : laterals with rather a deep framework, set 



Text-iie-ure 5. 



Lateral and rhachidian tooth of Mitra coriacea Reeve. 



with 10-12 sharp denticles along a straight upper margin, about 

 half of these are deeply rooted, they continue to the end, leaving 

 no bare space ; a small prow-like projection stands out from the 

 interior edge of the latei'al, pointing to the similar projection on 

 the rhachidian. 



In this group, of which coriacea is the single known repre- 

 sentative, the laterals are fairly typical, but the singular form of 

 the rhachidian separates it off. 



