362 ONUSTIDiE. 



MoUusks, but the form of their foot is admirably adapted 

 to the nature of the floor on which they live, which is 

 usually composed of the debris of dead shells. 



Genus ONUSTUS, Humphrey. 



Operculum sub-quadrate, elements elevated on the 

 upper surface at regular intervals, forming continuous 

 ridges radiating li'om the lateral nucleus. 



Shell trochiform, depressed, whorls flat, with foliaceous 

 or stellated margins, covered with fragments of shells and 

 stones near the sutures and apex, periphery surrounded 

 wdth tubulai* spines or radiating processes ; umbilicus 

 wide and deep ; outer lip much produced above, receding 

 far beneath. 



Ex. O. exutus. Reeve, pi. 40, fig. 1. Operculum, O. 

 exutus, fig. 1, rt, 1, b. Shell, O. Solaris, Linnceus,fig. 1, c. 



The thin, produced margins of the whorls, the wide 

 and deep umbilicus, and the very peculiar operculum, 

 serve to distinguish this genus. 



Species of Onustus. 



calculiferus, Reeve. Indicus, Mart. 



exutus, Reeve. Solaris, Lmn. 



Genus XENOPHORA, Fischer. 



' Operculum thin, ovate, elements simple, subannular, 

 nucleus lateral. 



Shell trochiform, concave beneath, irregular, whorls 

 fiat, more or less concealed by fragments of shells and 

 stones; spire depressed; aperture wide ; umbilicus small. 



