P L A C U N A. 



(Jenus PlaCUXA, Brtigruien'. 

 Testa bivaln's, discoidea, compressa, foliacea, radiatim 

 striata. Impressio miisculnris suhcentralif, circu- 

 lang. Margo cardinalh ralnilce sinistrm foveolis 

 diiabus divergentibnf nirtilaf/inifeiis, valvulue dextra; 

 dentibus duabus lainiiiatis divergentihus inimttis. 

 Slu'll bivalve, tliscoidal, coni])ressetl, foliaceous, radi- 

 ately striated. Muscular impression siibcentral, 

 circular. Hinge margin provided with two diverg- 

 ing cartilaginiferons gi-ooves in the left valve, and 

 two laminated diverging teeth in the right valve. 

 I'larenta. Retzius. 



The simple characters oi' the genus as above de- 

 scribed are sufficient to define it, excluding all other 

 shells and including the five species which are described 

 in this monograph. They are all flat bivalves of a 

 foliaceous and striated structure, k ith the two diverging 

 teeth in one valve, and correspondingly divergent liga- 

 ment pits in the other. The degrees of divergence in 

 these teeth and pits, although usefully observed as dis- 

 tinguishing species, cannot surely, with any advantage, 

 he made the basis of generic distinction. Yet this is 

 done hy some modern systematists, who actually liave 

 the genus Placenta for the common white species, and 

 the genus Placuna for the remaining species, without 

 any other reason. The sliores of China and India 

 furnish tlie two best known and widely distinguished 

 species, namely the "Chinese Window" and the 

 "Saddle Oyster." The third (J'laciiiKi piipt/racea) is a 

 somewhat dovibtftd species, partaking the characters of 

 the two first, and is probably common to India and 

 North Australia, while the remaining two, namely /*. 

 Liiicoliii iiui P. hihnla, the latter here described for the 

 first time, are exclusively Australian. 



Species 1. (Mus. Brit.) 



Pi..\crxA SKLLA. Pla. testa pleruiiujue ojmcd, intense 

 purpured ; crassiusculd, compressd, siibquadratd, 

 anricuhitd ; aiiriculis utrinqne recui-vis, medio 

 platndatim depresso, iiiargine ventrali recurro, ar- 

 cuato; marginibtis lateralibus decui-vis, Jlexuosis ; 

 mnrgine dorsnli lato ; viilvuld dextrd radiatim acute 

 striatd ; valmld sinistrd ad apicem vwiveiiusculd, 

 leviter striatd : dentibm laminifvnnibus submjiiali- 

 bns, late divergentibns. 



The saiidle Placuna. Shell for the most part intensely 

 imrjile, opaque, rather thick, compressed, sub- 

 quadrate, auriculated ; auricles on both sides 

 recurved; centre flatly depressed ; ventral margin 

 recurved, arclied ; lateral margins bent downwards, 

 fle.xuous ; dorsal margin wide ; right valve ra- 

 diately sharply striated ; left valve a little convex 

 at the apex, lightly striated ; lamiuiform teeth 

 nearly equal, widely divergent. 



Lamakck. Animaux sans Vertebres. Placuna No. 1. 



" Ephippium auglicunwn inaxiiniiiii," Cheumitz. 



Anomia sella. Gmelin. 



Placenta Ephippium, lietzius. 



Hab. Indian and Chinese Coasts. 



The peculiar saddle-like form into which this species 

 is bent by the curvature of its margins when fuU- 

 gi-own, does not begin to appear in the earlier stages. 

 The young shells are flat, and more or less transparent, 

 and are often of so light a colour as to be hardly dis- 

 tinguishable from the young of P. placenta or P. 

 \ papyracea. There are specimens of a beautifully 

 mottled, half La-own variety in the Cumingian Collec- 



