PARTS OF UNIVALVE SHELLS. 95 



Beak or rostrum, is that lengthened process in which the 

 canal is situate. This process is not so conspicuous in some of 

 the species of Voluta, but is more marked in the genera Murex, 

 Fusus, &c. 



The columella or pillar, is that process which runs through 

 the centre of the shell in the inside, from the base to the apex, 

 and around which the whorls or wreaths of the spire are wound 

 (fig. 19, page 34). When the columella is marked by ridges 

 or folds, as in Auricula (Jig. 31) and Oliva (Jig. 67), it is said 

 to be plicated or plaited, but when it is smooth, as in Jig. 116, 

 it is simple. 



Pillar lip, is a continuation of the glassy process with which 

 the aperture is lined, and expanded on the columella (figs. 58 

 and 66). It is also called the inner or internal lip (fig. 116). 



The outer lip, or external border, is the expansion or con- 

 tinuation of the body of the shell, on the left margin of the 

 aperture, and is also lined with the glossy process of the aperture 



(fig- H6). 



Spire consists of all the whorls of the shell except the lower 

 one, which, as before observed, is termed the body of the shell. 

 This spire is a prominent feature of the univalve ; and upon its 

 being elevated, depressed, &c., depends much of the generic and 

 specific definition of these shells. 



It is a remarkable circumstance, that many of the young 

 shells have not the same number of wreaths as the adults ; 

 from which it would appear, that the part of the animal nearest 

 the apex never increases in size. The number of wreaths 

 cannot, at all times, be depended upon. A full-grown shell 

 may, however, be known from the outer lip, whicl^has generally 

 an unfinished appearance in young shells. Indeed, in all the 

 land and fresh-water shells, it is a distinct criterion, as they are 

 never complete in the form of the outer lip, till full-grown. 



Whorl is one of the wreaths or volutions of the shell (fig. 

 116). 



Depressed spire is when the spire is very flat, as in the shells 

 of the genus Planorbis (fig. 29, page 42). 



Involuted spire, is where the spire is concealed in the inside 

 of the first whorl or body, as in some of the Nautili, Cvprsese, 

 Ovulie, &c. (fig. 68). 



Suture of the spire, or whorls, is a fine spiral line or seam, 

 formed by the joining together of the whorls ; it is sometimes 

 crenulated, undulated (waved), or sulcated (grooved), and not 

 unfrequently elevated or projecting (fig. 116). 



Reversed, or heterostrophe spire, is when the volutions of the 

 spire revolve in the same manner as the common corkscrew, or 



