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, uu; and Plate II. tig. 

 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 hmy, such 

 as most of the Myse, Solenes, Tellinae, &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 Mya 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 aculeutum. b. Lateral 



tooth, d d. middle teeth, //. spines. 



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

 marck.) 



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



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, 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, from the ventral margin to the imibones, is 

 what is above called the length. 



