P L A C U N A. 



Species o. (Fig. a, b, r, and Plate IV. fig. d, Mus. Brit.) 



Placun.\ i'lacenta. Ph. testdalbd, orbiculan, peUucidd, 

 co7iq>ressd, planulatd, margintbus rarb stibjlexuosis ; 

 dentibus laminiformibus longitudine valde inaiquaU- 

 hus, anguste divergentibus, valvuld dnistrd prope 

 vmbonem purpurea radiato. 



The placenta Placuna. Shell white, orbicular, pel- 

 lucid, compressed, flattened, margins rarely sub- 

 flexuous ; laminiform teeth very unequal in 

 length, narrowly divergent ; left valve rayed with 

 purple near the umbo. 



Linnaeus {Anemia) Systeuia Naturx. 



Placenta (irbinihin^. Hetzius. 



Hab. China, North-west Australia, Philippines. 



The prevailing form of this shell, commonly called 

 the " Chinese Window," is orbicvilar as in fig. d, Plate 

 IV., but the very large specimen given in the present 

 plate exhibits not only a deviation from that form, but 

 also a tendency to curve upwards at the ventral margin 

 a little after the fashion of the " Saddle Oyster." The 

 young, being purple-rayed, are not always easily dis- 

 tinguishable from the young of P. sella and P. papy- 

 racea liy the form of the shell. But the great distin- 

 guishing mark of the species is the unequal length and 

 narrow divergence of the teeth. 



November. 1871. 



