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This genus receives Mytilus hirundo, Linn, of which I do not know 
of any having been found fossil. 
CXLI. Perna. A flat irregular bivalve; the hinge formed of 
many parallel linear teeth, not articulating, arranged on a transverse 
straight line. 
In this genus, formed by Bruguiere, are placed Ostrea perna, 0. 
isogona, O. ephippium, O. picta, and 0. legumen, Linn. Here are 
likewise placed, by Bruguiere, two fossil shells found on the borders 
of the Rhine. 
Not having the opportunity of seeing the work in which those fossil 
shells are delineated, Perl. Naturf. No. 11, Tab. 9, Fig. 9, I am unable 
to judge how far they agree with the following shell, which appears to 
belong to this genus. . This shell is figured by M. Walch, Tome n. 
PI. d. v. The substance of the shell, which is very thick, is formed of 
innumerable thin plates, which are exceedingly brittle, but still pos- 
sessing a considerable portion of the original colour and brilliancy of 
the nacre. The hinge, which is nearly as wide as the shell itself, is 
flat, and formed by numerous longitudinal and parallel grooves. 
This shell was first found on the mountain Del Sappo, near Bologna, 
by Monti ; and, since then, a stratum of similar shells has been found 
in the mountain Audona, in Piedmont, by M. Allion. A represen- 
tation of this fossil is given Plate XV. Fig. 8, from a specimen which 
I obtained from Mr. Strange’s collection. From a more perfect shell, 
which was in the Leverian Museum, I conclude that the valve in this 
fossil has lost about half its length. 
Lamarck, who, we have seen, has followed Bruguiere in separating 
several new genera from the genus Ostrea of Linnaeus, has added the 
following new genus, formed from recent shells lately discovered in 
the Red Sea and in the Antilles. 
CXLI I . Crenatula. An irregularly-formed flat bivalve; closed, 
not giving passage to any byssus ; the hinge linear, excavated and 
crenulated by a row of small pits, which receive the ligament. 
