14 INTRODUCTORY 



are in general longer than broad. Plate I. fig. 5, v v. Plate II. 

 fig. 4, iy T'. 



Byssiis, 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, Pinnse, &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 species, 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 definition 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. 

 figs. 7, 8, 9, ^ ^ A. 



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,^. 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. 



