ACEPHALA TESTACEA. 263 



employed in the arts, and it is from the extravasation of this substance that 

 are produced the oriental or fine pearls, taken by the divers at Ceylon, in the 

 Persian Gulf, &c. 



PINNA, Linnceus. 



The Pinnae have two equal valves, forming a segment of a circle, or resem- 

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

 their sides. The animal, the CHIMERA, 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, Lin., which is moreover recognised by the valves being roughened 

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

 anchored by its byssus. In the 

 \ 



ARCA, Linn<zus, 



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 inter- 

 lock with each other, and, as in the subsequent genera, with two fasciculi of 

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

 valves, which serve to close them. 



THJGONIA, Bruguieres. 



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 Cardise in the form of their shell, and the 

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



The fossil Trigonise 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 a 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 serving to draw 

 out, direct, and place the byssus. They are commonly' known under the 

 generic name of Muscles. 



MYTILUS, Linnceus. 

 The true My tilt or Sea Muscles have a closed shell, with equal, convex, and 



