PARTS CV UNIVALVE SHELLS. 93 



The univalve shells are much more numerous than any others, 

 both in genera and species ; and it requires a considerable de- 

 gree of attention to discriminate many of the species, as they 

 run into each other so much. 



In the examination of univalve shells, the general outline or 

 contour of the whole shell is the first particular to be attended 

 to, as this leads to those distinctions necessary in the definition 

 of simple, spiral, or turbinated shells, or more strictly, accord- 

 ing to the Linnean method of discrimination, univalves with a 

 regular spire, and those without a regular spire. Univalve 

 shells are classified principally from the shape of the aperture, 

 taken in conjunction with the general shape of the shell ; from 

 the spire being lengthened or depressed, being with or without a 

 canal, the length of the beak and its direction, together with 

 the particular form of the outer or external lip : the colour of 

 shells only serves as a specific distinction, and, even in this 

 respect, cannot in all cases be depended upon, although, in 

 others, it is an unvarying test. The particular manner in which 

 the spots are disposed, frequently characterizes species. 



A pex, is the summit, tip, or highest part of a shell (Jig. 116). 



Base, is the opposite extremity from the apex (Jig. 116). In 

 shells with a beak or rostrum (as the Murex,^-. 64), it implies 

 the tip of such beak; in shells without a beak it is understood 

 to be the lower part, as before mentioned, opposite the apex. In 

 the Patella, and some others, the base of the shell is that part on 

 which it rests when it is laid on its mouth ; in the Teredo, and 

 similar shells, it is the wider end. 



Body of the shell, is the first or lower whorl of the spire, in 

 which the aperture is situated ; this whorl is generally longer 

 than the others (fig. 18). 



Front, of the shell, is that side where the aperture is situated. 



Back of the shell, is the opposite side to that in which the aper 

 ture is placed. 



The venter or belly, is the most prominent part of the lower 

 whorl or body, generally situated in the vicinity of the lip over 

 the aperture, and formed by the convexity of the aperture. In 

 general this term is only made use of in describing shells 

 whose body is large in proportion to the size of the spire (Jigs. 

 52 and 59). 



Sides of a shell, are the extreme edges of the shell, when 

 viewed either in front or from the back. The right side is the 

 one next the observer's left hand, when the shell is viewed in 

 front ; and the side with the aperture in it, is the left side. 



The conical univalve shell is generally spirally convoluted : 

 sometimes, as in the nautilus (jigs. 15 and 16), in the same 



2Y# 



