( 45 ) 



CHAP. XX. 



GENERIC CHARACTER OF UNIVALVES, 



WITH A 



Description of their Component Parts. 



Q. HAVING now completed a survey of bivalve 

 shells, I will thank you to describe to me the 

 component parts of univalves; and also from 

 what the generic character is derived. 



A. Willingly. The character which appears 

 to have guided Linnaeus in the formation of the 

 genera in univalves, was the exterior figure of the 

 shell, the columella, the aperture, and form of 

 the mouth. 



Q. Now will you have the goodness to give 

 me an explanation of the wreaths or whorls ? 



A. The parts you mention are the circumvo- 

 lutions of the spire around the columella. 



Q. What is the columella ? 



A. It is the pillar, or middle column, about 

 which the wreaths form their spiral circuit. 



Q. Describe the aperture, or mouth (apertura) 

 of a shell. 



A. It is, as the term implies, the orifice, en- 

 trance, or opening of the shell, that part through 

 which the animal protrudes itself. 



