ACEPHALA TESTACEA. 
91 
and ornamented witli 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 Pintadinse, 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 are closely united by a ligament 
along one of their sides. The animal, the Chimera, Poll, 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 ;|;. In the 
Arc A, Lin. ^ 
The valves are equal and transverse, that is to say, the hinge occu- 
pies the longest side. It is furnished with a large number of small 
teeth, whicli 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 T, 
p. 146, et seq. 
-f* M. Poli also calls it an abdominal trachea, just as erroneously as he applies the 
same name to the foot of the Pectines, See. 
J The whole genus Pinna may remain as it is in Graelin : 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 seej., and Sowerb., Gen. of Shells, No. 
XXVI. 
§ M. de Blainville forms his family of tlie Arcacea or Polyodontes, from the 
genus Arca. 
