04 



PARTS OF UNIVALVE SHELLS. 



plane, but more usually in an oblique direction. As a general 

 rule, the spiral univalve, if viewed in 

 Base. the position in which its inhabitant 



would carry it, if moving forwards 

 from the observer, is twisted, from the 

 apex downwards, from left to right, 

 the spire being directed obliquely to- 

 wards the right. The annexed figure 

 (Jig. 116) shows the involutions or 

 whorls of the spire of the Pleurotoma. 

 In some genera, for example, Clausilia 

 (from the Latin, clausus, shut, jig. 25, 

 page 40) and Physa (from the Greek, 

 phusa, a bubble, fig, 30, page 42), the 

 shell is twisted in an opposite direc- 

 tion : such shells are called " per- 

 verse," or " sinistral." 



The aperture or mouth, is that part 

 of the lower whorl or body through 

 which the animal protrudes itself. 

 This is one of the principal means of 

 distinguishing the genera of univalve 

 shells, and it varies much in its form; 

 some apertures are rounded (figs. 43 

 and 46); others semilunar (fig. 55); 

 others angular (fig. 37), &c. Some 

 apertures have a canal at their base 

 (figs. 63 and 11 6), and others are with- 

 out it (fig. 51). In various genera, 

 the aperture extends the whole length 

 of the shell, as in Ovula (fig. 68), 

 Cyprsea, and some of the Cones with 

 depressed spires (fig. 70). In several individuals, the aperture 

 is either entirely open, or closed by an operculum or door, which 

 is usually affixed to the foot of the animal. 



When without a notch or canal, the aperture is said to be 

 entire (fig. 51). The aperture has two lips or borders; the 

 internal lip, or border, is on the side of the aperture, formed by 

 the columella, and the external, or outer lip, or border, is oppo- 

 site, as in the Pleurotoma (fig. 116) ; the letter b. indicates the 

 pallial notch ; n. notch of the syphon. 



Canal, or gutter (fig. 116), is the space or hollow, formed by 

 the prolongation of the two lips of the aperture. Some shells 

 have two canals, one situated at the point where the outer lip and 

 body join. 



Apex. 

 Fig. 116. PLEUROTOMA. 



