276 MOLLUSCA. 



bling a half-opened fan, which are closely united by a ligament along one 

 of their sides. The animal, the CHIM^KA, Poli, is elongated, like its shell; 

 the lips, branchiae, and other parts are in the same proportion. 



The byssus of several species of Pinna is as fine and brilliant as silk, and 

 is employed in fabricating the most precious stuffs. Such is the 



P. nobilis, L., which is moreover recognized by the valves being rough- 

 ened with recurved and semitabular plates. It remains half buried in the 

 sand, and anchored by its byssus. In the 



ARCA, Lin. 



The valves are equal and transverse, that is to say, the hinge occupies the 

 longest side. It is furnished with a large number of small teeth, which in- 

 terlock with each other, and, as in the subsequent genera, with two fasci- 

 culi of transverse and nearly equal muscles, inserted into the extremities of 

 the valves, which serve to close them. 



TRIGONIA, Brug. 



Remarkable for the hinge, which is furnished with two plates en chevron, 

 crenulated on both faces, each of which penetrates into two cavities, or 

 rather between four plates of the opposite side, similarly crenulated on their 

 internal surface. 



The living Trigoniae resemble the Cardiac in the form of their shell, and 

 the ribs which furrow it: its interior is composed of nacre. 



The fossil Trigoniac are different. Their shell is flattened on one side, 

 oblique, longest in a direction perpendicular to the hinge, and traversed in 

 a contrary direction by series of tubercles. 



FAMILY II. 

 MYTILACEA. 



In the second family of the testaceous Acephala, the mantle is 

 open before. 



All these bivalves have a foot, used in crawling, or at least serv- 

 ing to draw out, direct, and place the byssus. They are commonly 

 known under the generic name of Muscles. 



MYTILUS, Lin. 



The true Mytili or Sea-Muscles have a closed shell, with equal, convex and 

 triangular valves. One of the sides of the acute angle forms the hinge, and 

 is furnished with a long, narrow ligament. The head of the animal is in the 

 acute angle; the other side of the shell, which is the longest, is the anterior 

 one, and allows the passage of the byssus; it terminates in a rounded angle, 



