CONCHOLOGICAL TEEMS. 241 



The back, fig. 3, m, is the side directly opposite to that in 

 which the aperture is placed. 



The varices, fig. 3, n, are ribs which cross the volutions 

 in some species of buccinum, murex, and triton. They are 

 formed by the periodical growth of the shells ; the varices 

 being the former margin of the outer lip, to which the animal 

 has attached its successive enlargements. 



The columella, or pillar, fig. 4, 0, is that process which 

 runs through the centre of the shell from the base to the 

 apex, and is formed by the inner sides of the volutions of a 

 spiral univalve. It constitutes an important feature in these 

 shells ; on the mode in which it is striated, grooved, folded, 

 or otherM r ise marked, depend the generic and specific dis- 

 tinctions of many shells. It also forms the axis of revolution 

 around which the volutions are turned. In consequence of 

 the heart and great blood-vessels being usually placed on 

 the left of the shell, the turns are commonly made from 

 right to left ; on the contrary, where the heart and blood- 

 vessels are placed on the right side, the volutions are 

 reversed, and proceed from left to right. In the first 

 instance, the shell is termed dextral ; in the latter, it is 

 called sinistral, or reversed. 



The septa, or chambers, fig. 5, p, are the cavities, divided 

 by partitions, as in the nautilus, spirilla, &c. 



The umbilicus, fig. 6, q, is a circular perforation formed in 

 the base of many univalve shells, when the inner side of 

 the volutions do not join each other, so that the axis is 

 hollow. Such is the case in the genus trockus, in some 

 species of which it penetrates from the base to the apex. 

 Those shells which have no umbilicw are termed imper- 

 forate. 



BIVALVED SHELLS. Pig. 2 represents a bivalve shell 

 placed in its right position, which is that in which the 

 animal is supposed to be walking along the bed of the sea, 

 by means of its foot ; the opening of the valves through 

 which the foot protrudes is the ventral margin, and the 

 opposite, the back, or dorsal margin. If the animal is walk- 

 ing forward, with its back to the observer, the right and the 

 left valves will correspond with his right and left sides ; in 

 fig. 2 the shell is represented in this position ; therefore d 

 is the right valve and c the left. 



