170 CLASS I. TROPIOPODA. ORDER II. UNIMUSCULOSA. 



cardinalium affixum, utrinque partim extensum. Impressio muscu- 

 lans centralis. 



The genus Placuna, originally confounded by Linnaeus and his fol- 

 lowers with Anomiu, was introduced by Bruguiere. Its shell, though 

 easily recognised by the thinness and flatness of the valves, is particularly 

 distinguished by the peculiar construction of the hinge. It consists of 

 two distinct ribs or ridges, on one valve only, diverging upwards ; and to 

 the outer edge of these is attached the ligament. The valves are of the 

 same thin fibro-laminar structure as those of the Ostrea ; and although 

 the shell of that genus does not exhibit any indication of this new car- 

 dinal development, it is very characteristic in the genus Placunanomia, 

 which follows. We cannot agree with Deshayes in considering the car- 

 dinal ridges of the Placun?e as a modification of the bony organ of adhe- 

 sion in the Anomice, because these two parts are destined to perform 

 separate offices, and are both distinctly exhibited in the new and re- 

 markable genus just alluded to. 



The shell of Placuna may be described as being free, fibro-lairiinar, very 

 thin, smooth, transparent, orbicular, and nearly equivalve. The hinge 

 consists of two narrow ribs, unequal in length, converging at the base in 

 one valve ; whilst in the other are merely the corresponding marks. The 

 ligament runs along the outer edge of these cardinal ridges, extending 

 partially on both sides of the umbones. 



The valves of the following species are so thin and transparent, that 

 they are used by the Chinese for the purpose of glazing windows, lan- 

 terns, &c, in the same way as horn is sometimes used in this country. 



Example. 

 PI. CXXII. Fig. 1 and 2. 



Placuna placenta, Lamarck, Anim. sans vert., new edit., vol. vii. p. 270. 



Enc. Meth. pi. 1 73. f. 1 and 2. De Blainville, Manuel de Malacologie, 



pi. 60. f. 3. 

 Anomia placenta, Linnaeus. 



