PLACUNID^. 295 



DiPLOSCHiZA, Conrad, 1866. 



Distr, — D. cretacea, Conr. (cxxxiii, 27, 28). Cret. ; Ala. 



Subovate, inequivalve, smaller valve concave, both of a lami- 

 nated structure and with truncate, deeply notched or emarginated 

 beaks. 



Conrad sa^'-s that *' the shell seems to have been attached by 

 the umbo of the larger valve, the truncature of which reminds 

 us of the truncated beak of Terebratula." It is a curious shell 

 and indicates great similarit}' to a brachiopod form. 



OsTRENOMiA, Conrad, 1812. 



Distr. — 0. Carolinensis^ Conr. (cxxxiii, 29, 30\ Eocene ; 

 N. Carolina. 



Shell inequivalve, irregular, latninated ; hinge with a trian- 

 gular cartilage-pit; right valve with a deep notch or sinus 

 having an internal raised margin; left valve with an angular 

 dentiform process at the base of the cartilage-pit. 



Carolia, Cantraine, 1835. 



Dedicated to Prince Charles Bonaparte. 



Syn. — Hemiplacuna, G. Sowerb^' . 



Distr. — 3 sp. Tertiary ; Egypt. C. placunoides., Cantr. 

 (cxxxii, 2). 



Shell like Placuna ; hinge, when young, like Anomia, with a 

 byssal plug passing through a small deep sinus in front of the 

 cartilage-process, which is closed in the adult. 



Family PLACUNID^. 



Shell equivalve or nearly so, compressed, thin, pearly, exter- 

 nally^ often finely lamellar ; ligament marginal, cartilage attached 

 to the external side of two diverging ribs in one valve, corres- 

 ponding to two similar grooves or ribs in the other valve. 



Free, without byssus, found on sandy shores. 



Plaouna, Solander. 



Etym. — Plakous, a thin cake. Window-shell. 



Distr. — 4 sp. Scinde, North Australia, China. P. orbicularis, 

 Retz. (cxxxiii, 31). 



Shell suborbicular, compressed, translucent, free, resting on 

 the right valve ; hinge-area narrow and obscure ; cartilage 

 supported by two diverging ridges in the right valve and cor- 

 responding grooves in the left ; muscular impressions double, 

 the larger element round and central, the smaller distinct and 

 crescent-shaped, in front of it. 



The Placunge are very closely allied to Anomia ; and many 

 intermediate forms may be traced. The shell of each consists 



