CATALOGUE OF 
placentadj: & anomiad^. 
Family PLACENTAD.E. 
Placentadae, Gray, Proc. Zool. Soc. 1848, 201. 
PlacuuidcB, Gray, Syn. B. M. 1842, 84, 92 ; Hermann. Ind, 279. 
Placunoidse, Agassiz, Nomen. Zool. 1847 ; Hermann. Ind. 279. 
Placiina, Lamk. Hist. 2 ed. vii. 269. 
Ostrea, part Ferns. Tab. Syst. 40, 1821. 
Pectiiiidae, part Fleming, Brit. Anim. 381, 1828. 
Anomia, part Hermannsen, Ind. i. 61, 1846. 
Lamarck describes three species of this f^eiius, depending on the 
general outline and the waved or flat form of the shell, characters 
which are liable to considerable variations, as may be found on the 
mere inspection of any considerable number of specimens. 
The hinge forms a more permanent character, and affords the 
means of dividing the species into two sections, and furnishes cha- 
racters which separate them from each other. The right valve is 
the flattest, and bears the ridges of the hinge. 
Chemnitz gives the best character for the species, and has ob- 
served the character furnished by the hinge, which has been over- 
looked by Lamarck, and by all recent authors. 
Synopsis of the Genera. 
a. Hinge-ridges linear, diverging, only slightly raised. 
1. Placuna. Hinge-ridges of nearly equal length ; muscular scar 
under centre of hinge. 
2. Placenta. Hinge-ridges unequal, hinder much the longest ; 
muscular scars rather in front of middle of hinge. 
B 
