24 INTRODUCTION. 



beak, to the 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, u u ; and Plate II. fig. 

 4, u u. 



Breadth is measured fro.m the most extreme eCi^e of the 

 anterior and posterior slopes, being in a contrary direction 

 from its length. Many shells are broader than long, such 

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

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

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



Byssus, or heard, 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 LINNiEAN GENERA OF BIVALVES. 



Fig. 1 — Hinge of the JJT/a pictoruvi. (Unio pictorum, 

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



Fig. 2. — Hinge of the Solen Siliqua. b. Lateral teetli. 



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



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



Fig. 4 Hinge of the Cardimn aculeutum. b. Lateral 



tooth, d d. middle teeth, II. spines. 



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

 marck.) 



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



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 lengtli of the shell, however, is in the direction of the 

 length of the animal, or from the anterior to the posterior extremity 

 of the shell, and its height, ft'om the ventral margin to the umbones, is 

 what is above called the length. 



