140 



Figure. 



Trochiscus Norrisii. PI. 14. Fig. 68. Shell, showing its smooth solid 

 growth, large deep umbilicus and operculum. — From Mr. Cuming's 

 collection. 



Genus 10. MONODONTA, Lamarck. 



Animal ; similar to that of Turbo. 



Shell ; rather small, ovate or conoidal, rather solid, columella 



excavated, truncated and toothed at the base, lip sometimes 



den lately ridged within, margins disjoined. 



The genus Monodonta was introduced by Lamarck for the sake of dis- 

 tinguishing a numerous group of Troc/d, from the typical forms of that 

 genus, on account of the columella being suddenly truncated in a manner 

 so as to present a tooth. It has not, however, been generally adopted on 

 account of the gradual development of this peculiarity commencing in 

 Trochus, and rendering it difficult to say exactly where the genus under 

 consideration should commence. In treating of the Turbinacea generically 

 in a former work, I came to the same conclusion as M. Deshayes, that the 

 genus Monodonta is unnecessary ; but upon coming to an examination of 

 the species, so extensive and characteristic a series being conveniently sepa- 

 rated under this head, I have not hesitated to follow Lamarck's arrange- 

 ment of the series for facility of reference. 



For List of Species see Appendix. 



Figure. 



Monodonta labeo. PI. 13. Fig. 64. Shell, showing a characteristic 

 development of the tooth upon the columella. — From Mr. Cuming's 

 collection. 



Genus 11. MORULUS. 



Animal -, ? 



Shell ; extremely variable in form, sometimes ovately turbinated 

 with the apex depressed, sometimes Trochi-form, raised towards 

 the apex into a sharp pyramid, of a peculiar opake white sub- 

 stance more or less stained with brown ; columella distinguished 

 by a depressed lamettiform tooth. 



