RADULA OF THE UITRIVJE. 419 



not deeply bowed, tips of framework bare; denticles 10-11, 

 central slightly the largest : blade of laterals narrow, somewhat 

 angled in the middle. 



Group (2). Rhachidian tricuspid, cusps small, placed close 

 together in the centre of the curve, all the rest of the framework 

 bare of cusps. 



Text-fiofure 18. 



/=^=v= 



Lateral and rliacliidian tooth of VexiUtun australe Swains. 



1. V. austi'ale Swains. IS". Tasmania. The specimen is in bad 

 condition, but enough can be seen to testify that the rhachidian 

 is tricuspid, cusps somewhat large, central slightly the smallest : 

 laterals thickish below, narrowing sharply at the ends. 



2. F. e6e?ms Lam. Malta. Fi-amework of rA«cA«cZmw narrow, 

 deeply arched, but scarcely waved in bow shape ; cusps very close 

 together, thin, sharp, equal in length, central slightly the 

 narrowest : blade of laterals narrow, slightlj^ hooked at tip. 



3. hizenense Pils. Japan. Framework of rhachidian deeply 

 arched in bow shape ; denticles smaller and somewhat further 

 apart than usual in this group, equal in size : blade of laterals 

 long and rather narrow. 



4. V. porphyreticum Reeve. Tonga. Rhachidian with frame- 

 work deeply waved, cusps small, equal-sized, close together : 

 blade of laterals deeply narrow, angularly curved in centre. Rows 

 7t)-l- nascent. 



A general survey of the Mitridan radulte suggests several con- 

 siderations. 



In the first place, the rhachidian tooth exhibits wide differences 

 of structure, ranging from the lozenge-shaped 8-9-cusped form in 

 Mitra, to the unicuspid triangular form of the variegata group. 

 It is probable that the investigation of fvirther material may 

 discover links between forms of Mitridan rhachidians at present 

 very dissimilar. But there appears to be warrant for the sug- 

 gestion that the Mitridse represent an ancient groujD of Mollusca*, 

 so that many links in the chain of development may have become 

 extinct. 



Secondly, these divergencies in the structure of the rhachidian 

 are accompanied by a general similarit}) of plan in the laterals, 

 subject however to a progressive modification in their form. 

 "We can arrange the groups and sometimes the species in a 



* Mr. K. U. Newton informs nie that Bellardi enumerates nearly' 200 species of 

 Mitridse from the Upper Tertiaries of Italy alone, and that the genns dates back to 

 Cretaceous rocks. 



Piioc. ZooL. Soc— 1919, Xo. XXIX. 29 



