24 INTRODUCTION. 



beak, to tte opposite margin. For example, the common 

 muscle: it is longer than it is broad, and the solens are 

 broader than long. Plate I. fig. 5, uu; and Plate II. fig, 

 4, u u. 



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 luny, such 

 as most of the Myae, Solenes, Tellinse, &c. ; and the genera 

 Mytilus, Ostrea, Pinna, &c., are in general longer than 

 broad. Plate I. fig. 5, v v. Plate II. fig. 4, v v. * 



Bi/ssus, 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 Muscles, Pinnae, &c. 

 Plate II fig. 6, s. 



DESCRIPTION OF PLATE IV. 



HINGES OF THE LINN^AN GENERA OF BIVALVES. 



Fig. 1 — Hinge of the M;/a pictorum. (Unio pictorum, 

 Lamarck.) a. Primary tooth, c c double teeth. 



Fig. 2 Hinge of the Solen Siliqua. b. Lateral teeth. 



Fig. 3 Hinge of the Tellina Radula. (Lucina radula, 



Lamarck.) a a. Primary teeth, n. cartilage. 



Fig. 4 Hinge of the Cardium aculeatum. b. Lateral 



tooth, dd. middle teeth, II. spines. 



Fig. 5 — Hinge of Mactra Mans. (Lutraria hians, La- 

 marck.) 



Fig. 6 Inside of both valves of the Donax Trunculua. 



A is the right valve, and B is the left valve ; b. lateral 

 teeth, e. primary complicated tooth, or cleft in the middle, 

 m m. crenulated margin. 



» The true length of the shell, howeA'er, is in the direction of the 

 length of the animal, or fi-om the anterior to the posterior extremity 

 of the shell, and its height, fi-om the ventral margin to the umbones, is 

 what is above called the length. 



