MOLLUSCA. 89 



Trans.) The first important improvement in the reforma- 

 tion of the genus, consisted in the separation of the pec- 

 tens, which was executed by Pennant, and afterwards by 

 Bruguiere and Lamark. Since new characters have been 

 assigned to the genus ostrea, other separations must take 

 place. It is thus defined : " Testa bivalvis inaequivalvis, 

 rudis adhaerens ; cardine edentulo. Fossula cardinalis ma- 

 joris valvae setate crescens. Ligamentum semi-internum. 

 Impressio muscularis unica." The genus may be divided 

 into two sections ; the first having the margin of the valves 

 simple, as the common oyster ; and in the second the mar- 

 gins are plaited, as in the O. crista-gallL 



In consequence of this change in the generic character, 

 the ostrea malleus of Linnaeus, (Lister, tab. 219, f. 54.,) has 

 been formed into the genus MALLEUS. The shell is free, 

 gapes a little at the beaks, produces a byssus, has no teeth 

 in the hinge but a conical pit for the insertion of the liga- 

 ment, placed obliquely on the margin of each valve. It 

 was for a long time highly prized by collectors. 



The genus PECTEN is one of the best characterised, most 

 natural, and most beautiful, in the system. The shell is 

 inequivalve and regular, the hinge is destitute of teeth, and 

 the internal ligament is fixed to a triangular cardinal cavity. 

 There are fourteen species natives of our shores. 



From the ostrea perna, ephippium^ and isogonum, Bru- 

 guiere formed the genus PERNA. The hinge is linear, and 

 cut into a number of lengthened parallel veins, which re- 

 ceive the ligament. The interstices are formed into teeth, 

 which simply oppose those of the* other valve. In the an- 

 terior side of the valve, near the beaks, there is a callosity, 

 and an opening for the byssus of the animal. 



Lamark has constituted another genus, nearly allied to 



