OF CONCHOLOGY. 39 



or other form. Plate 3d, fig. 6th, W. 



Umbilicus is in general a circular 

 perforation, in the base of the lower 

 whorl, or body, of many univalves ; and 

 common to most of the Trochi, in 

 some of which it runs from the base 

 to the apex ; widest at the base, and 

 gradually tapering to the top. Plate 

 3d, nV 1st, U. 



Sub-umbilic cited shells, are those 

 which have the umbilicus covered in a 

 greater or less degree by a thin pro- 

 cess ; which, in some, almost entirely 

 closes the opening or mouth. 



Shells which have no umbilicus are 

 termed imperforate. 



Siphunculus is that small round 

 perforation which forms a communi- 

 cation between the chambers of the 

 Nautili, which runs through the whole 

 spire of the shell. Plate 3d, fig. 11th, 

 V. 



Ribs are those longitudinal protu- 

 berances which are in many of the 



