14 INTRODUCTORY 
are in general longer than broad. Plate I. fig. 5, vv. Plate II. 
fig. 4, vv. 
Byssus, or beard, is an appendage composed of filaments of a 
silky texture, by which some of the Bivalves fasten themselves 
to their beds, such as the Mytilus, Pinne, &c. Plate II. fig. 6, s. 
UNIVALVE. 
The shells called univalve, or those composed of one part only, 
are far more numerous than the two preceding, both in genera 
and species ; and it requires a considerable degree of attention to 
discriminate many of the sfeczes, as they run into each other so 
much ; and they are divested of the strong and distinct character 
afforded by the teeth of bivalves; besides many of the species 
there are several varieties. 
In the examination of shells of this order, the general contour 
or outline of the whole shell is the first particular to be attended 
to, as this leads to those distinctions necessary in the defimition of 
simple, spiral, or turbinated shells, Univalves with a regular spire, 
and those without a regular spire. ‘The genera of this order are 
formed principally from the shape of the aperture, taken in con- 
junction 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 lip: the colour of shells only serves as a specific 
distinction, and cannot in this respect, in all cases, be depended 
upon, although in others it is an unvarying test. The particular 
manner in which the spots are disposed frequently characterises 
species. 
Apex. The summit, tip, or highest part of the spire. Plate II, 
HES MANS, 9,104 wh wt, 
Base is the opposite extremity from the apex, or tip of the spire. 
In shells with a beak it implies the tip of such beak; Plate II. 
fig. 11, B. In shells without a beak it is understood to be the 
lower part, as before-mentioned, opposite the apex ; Plate II. figs. 8 
and 9, B B. 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 Dentalium and Teredo it is the wider end. 
