RADULA OF THE MITRID^. 421 



of extra intei'vening cusps. In most cases of bicuspid laterals, 

 one or other of the cusps, usaallj^ but not always the interior, is 

 decidedl}' the smaller of the two. This smaller cusp may show 

 signs of incipient disappearance, a stage ^yhich receives illustra- 

 tion from Troschel, pi. vii. figs. 15, 19, whei-e the inner cusp of 

 11 Hemifasus and a Myristica is very small, and pi. viii. figs. 5, 6, 

 Avhere the outer cusp of a Vasum is similarly reduced. It is no 

 .great stejj from laterals of this type to forms in whicli the second 

 cusp has vanished altogether. The framework of the tooth 

 would diminish in breadth, the fewer cusps it had to carry. 



This process of reduction in the rhachiglossate latei\al seems to 

 receive illustration from the case of the Volutidfe. Most species 

 of Voluta have lost their laterals altogether. Valuta concinna 

 and Volutilithes ahyssicola alone retain them in a very degraded 

 form, unicuspid on an oblong base. A further stage is shown in 

 Neptnneojisis gilchristi, where the lateral cusp has vanished, and 

 the framework is reduced to a small thin plate, of undefined shape, 

 ■" probably quite functionless " (Pace 7, Woodward, M. P. 14). 



In other genera the process of degradation takes another 

 line. The laterals remain unimpaired, while the rhachidian 

 suffers degradation, or even disappears altogether. Fasciolaria, 

 Latirus, Peristernia, and Fasinus form a group of genera in which 

 the rhachidian is reduced to extreme smallness, while the rhachi- 

 dian cusps liRve in many species almost vnnished. It cannot be 

 doubted that the rhachidians of C'olumbella and of Liomesus 

 {^=Bucsinopsis),which are now'mere narrow blocks, are descendants 

 of teeth which once were furnished with cusps. 



Bihliography . 



1. Fischer, P. — Manuel de Conchyliologie. Paris, 1887. 



2. Garrett, A. — "Catalogue of the Polynesian Mitridfe, with 



remarks on their .... station.'" Journ. of Conchol. iii. 

 pp. 1-73, 1880. 



3. Gray, J. E. — "On the division of Ctenobranchous Gastero- 



podous Mollusca into larger Groups and Families." Ann. 

 Mag. Nat. Hist. ser. 2, xi. pp. 124-133, 1853. 



4. Gray, J. E. — " On the Teeth of the genus Mitra, Lamarck." 



Ann. Mag. Nat. Hist. ser. 2, xii. pp. 129-130, 1853. 



5. Hogg, J. — "The Lingual Membrane in Mollusca, and its 



value in classification." Trans. Roy. Micros. Soc. xvi. 



pp. 93-104, pi. x. f. 37, 1868 {Mitra fasca Swains.). 

 '6. Macdoxald, Dr. — Letter in Trans. Roy. Soc. Mauritius, n. s. 



V. p. 109, 1871 [Mauritia harclayi H. Ad.). 

 7. Pace, S. — "On the Ana.tomy and Relationships of Voluta 



viusisa Linn., with Notes upon certain other supposed 



members of the Volutidte." Proc. Malac. Soc. London, 



V. pp. 21-31, 1902. 

 -8. Sars, G. 0. — Mollusca Regionis Arcticfe Norvegica?. Christi- 



ania, 1878. 



29* 



