24 THE ELEMENTS 



inside. Plate 4th, figures 3d, 6th, Yth. 

 9th, &c. 



Length of the Shell is taken from 

 the ligament, or the beak, to the 

 opposite margin. Plate 1st, fig. 5th; 

 and plate 2nd, fig. 4th, uuuu. 



Breadth is measured from the most 

 extreme edge of the anterior and 

 posterior slopes, being in a contrary 

 direction from its length. Many shells 

 are broader than Ions', such as most 

 of the Myse, Solina, Tellina, &c. ; and 

 the Mytilus, Ostrea, Pinna, &c. are in 

 general longer than broad. Plate 1st, 

 fig. 5th; plate 2nd, fig. 4th, vvvv. 



Byssus, or heard, is an appendage 

 composed of filaments of a silky tex- 

 ture, by which some of the Bivalves 

 fasten themselves to their beds, such 

 as the muscle, Plate 2nd, fig. 6th, s. 



UNIVALVE. 



The shells composing this order, 

 are far more numerous than the two 



