ACEPHALA TESTACEA. 91 



and ornamented with the most beautiful nacre within. The lat- 

 ter is 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. 



The name of Avicula is appropriated to such as have more pointed 

 ears, and a more oblique shell. The vestige of a tooth, of which 

 traces are visible in the Pintadinae, is observed on the hinge, before 

 the ligament. 



One species, Mytilus hirundo, L., Chemn., VIII, Ixxxi, 722 — 

 728, that inhabits the Mediterranean, is remarkable for the 

 pointed ears which extend its hinge on each side. Its byssus is 

 coarse and stout, resembling a little tree *. 



Pinna, Lin. 



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

 resembling a half opened-fan, which arc closely united by a ligament 

 along one of their sides. The animal, the Chim;era, Poli, is elongated, 

 like its shell ; the lips, branchiae, and other parts are in the same 

 proportion. The mantle is closed along the side of the ligament ; 

 the foot resembles a little conical tongue excavated by a sulcus ; it is 

 furnished with a small transverse muscle situated at the acute angle 

 formed by the valves, near which is the mouth, and with a very 

 large one in their broader portion. By the side of the anus, which 

 is behind this large muscle, is a conical appendage, peculiar to the 

 genus, susceptible of expansion and elongation, the use of which is 

 unknown f . 



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. nohilis.'L., Chemn. VIII, Ixxxix ; which is moreover re- 

 cognized by the valves being roughened with recurved and semi- 

 tabular plates. It remains half buried in the sand, and anchored 

 by its byssus X- In the 



ArcAj Lin. § 



The valves are equal and ti-ansverse, that is to say, the hinge occu- 

 pies the longest side. It is furnished with a large number of small 

 teeth, which interlock with each other, and, as in the subsequent 

 genera, with two fasciculi of transverse and nearly equal muscles, in- 



* Several species are now made of it. See Lam., An. sans Verteb., VI, part I, 

 p. 146, et seq. 



t M. Poli also calls it an abdominal trachea, just as erroneously as he applies the 

 same name to the foot of the Pectines, &c. 



+ The whole genus Pinna may remain as it is in Gmelin : it is well to remem- 

 ber, however, that some of his species may be found to form but one. See also 

 Lam., An. sans Vert., VI, part I, p. 130, et seq., and Sowerb., Gen. of Shells, No. 

 XXVI. 



§ M. de Blainville forms his family of the Arcacea or Polyodontes, from the 

 genus Arca. 



