420 DE. A. H. COOKE ON THE 



group (cf. Inihricaria), in a, series indicating a transition from a 

 more complex to a simpler form of tooth. The laterals, in fact, 

 exhibit eveiy symptom of regress towards a gradual degradation. 

 At the head of the series we have the multicuspid lateral 

 of Mitra proper, Fapalaria, and Dihajjhus, in which the 2nd, 

 3rd, and 4th cusps are slightly larger than the rest, and incline 

 sliglitly inwards. The first important modiiication occurs in 

 StriijateUa, where, on the outer half of the lateral, the cusps 

 either diminish to mere dots, or are absent altogether, leaving the 

 framework bare. In Tmhricaria (a. parallel development) one or 

 more of the interior cusps increases considerably in size, at the 

 expense of the others, which generally become smaller, or are 

 reduced in numbers and become degraded. 



In thejla7nmigera and S2)hce)-i(.lata groups further modification 

 occurs. As a rule, one of the lateral cusps is enormously 

 developed, with a corresponding decrease in size, oi- with the 

 total disappearance, of the remaindei' of the cusps, tlie whole 

 tooth assuming a degraded appearance. In the variegata group 

 the big lateral cusp has ousted all the others, and the tooth is 

 mvicii degraded. 



According to Troschel, Cyl'mdromitra (formerly C'l/lmdra) 

 nucea Meusch. has no latei'als, while specimens of C)/. dactylus L. 

 have been examined both by himself and Dr. Gray withoitt any 

 trace of a radula being discovered. It is possible therefore that 

 Gylindromitra forms the last term in the series of degraded foi'ms 

 of Mitridan radula; on the other hand, it may be wiser to wait 

 for fux^ther evidence before a final decision can be reached. 

 Troschel's figures can seldom be accepted without confirmation. 

 He places the Ziervogelia group of Strigatella close to VexiUum. 

 If the radulas he figures are correct, Ziervogelia cannot be regarded 

 as akin to Strigatella ; but here again further evidence is needed 

 before the true position of Ziervogelia can be settled. 



It is tempting to pursue this branch of the subject a little 

 further, and to enquire vv'hether, in the progressive degradation 

 of the Mitridan lateral, we can obtain a clue to the genesis of 

 the familiar bicuspid or unicuspid lateral of many of the lihachi- 

 glossa. It seems within the bounds of possibility that the 

 coalescing, or gradual disappearance, of the cusps, in a multicuspid 

 latei-al, produced, in more cases than that of the Mitridae, a lateral 

 with one or two large cusps instead of many small ones*. In 

 Buccinuvi, Neptunea^ Cominella, Euthria, and the allied genera, 

 small cusps occur, witli some degree of irregularity, between the 

 two, or three, great cusps. It is conceivable that these smaller 

 cusps may be vestiges of an armature, in which the cusps were 

 more numerous and more on a level as regards size. Even a 

 genus like Alecfrion, which has settled down, in most species, to 

 a. steady bicuspid lateral, occasionally exhibits more than traces 



* Compare B. B. Woodward, Proc. Malac. Soc. London, vii. p. 259 : " In the 

 Kadula . . . there is consistent progress in the shape of the replacement of nume- 

 rous, weak, little teeth h}' few strong ones, especially in the carnivorous groups." 



