26 INTRODUCTORY. 



Chambers are the cavities divided by partitions, at regular or 

 irregular intervals; as in the Nautilus, Plate III. fig. U, w w 

 w w w. 



In sooie of the Serpula there are also divisions, but they are 

 not regular as in the Nautili ; and besides, they differ from them 

 in being devoid of a siphunculus or communication between the 

 chambers ; the animal forms a complete partition and adds to 

 its shell, which it would appear to be necessitated to do, from its 

 body growing too large for its abode. 



Several of the Patellce have chambers formed of laminous 

 partitions, subspiral cells, or processes ; these in general lie hor- 

 izontally, and are quite open at one end, as in the Patella testu- 

 dinaria, &c. of Linni3eus, and the prickly Crepidula, and Chinese 

 Calyptrffia, plate XIV. fig. 16 and 19 ; and plate III. tig. 6, ir., 

 or funnel shaped, as in the Sypho striates. Plate XIV, fig. 21, 



Umbilicus is in general a circular perforation in the base of 

 the lower whorl, or body, of many univalve shells. This is com- 

 mon to most of the Trochi, in some species of which it pene- 

 trates from the base to the apex ; widest at the base, and gradu- 

 ally tapering to the top. Plate III. fig. 1. u. 



Sub'Umbilicated shells, are those which have the umbilicus 

 covered in a greater or less degree by a thin process ; which, in 

 some, almost entirely closes the opening or mouth. This cha- 

 racter is most commonly to be met with among species of Bucci- 

 num and Murcx. 



Shells which have no umbilicus are termed imperforate. 



Siphunculus is that small round perforation which forms a 

 communication between the chambers of the Nautili, and pene- 

 trates through the whole spire of the shell, Plate III. fig. 11. v. 



Varices are transverse ribs which cross the whorls of shells in 

 some species of Buccinum, Murex, and Tritonia, and exempli- 

 fied in the Cassis areola, Plate XL fig. 21. Varices are 

 formed by the periodical growth of the shells, these being the 

 margin of the outer lip, to which the animal has attached its 

 periodical enlargements. In some species they have more the 

 form of sutures than ribs ; this is owing to the margin of the 

 outer lip being but slightly developed. 



Ribs are those longitudinal and transverse protuberances 

 which are in many of the univalve shells. Plate III. fig. \2, 

 r r T Ts 



