PLACUNIDA. 295 
Diptoscuiza, 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 says 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 similarity to a brachiopod form. 
OstRENOMIA, Conrad, 1872. 
Distr.—O. Carolinensis, Conr. (cxxxiii, 29, 30). Eocene; 
N. Carolina. 
Shell inequivalve, irregular, laminated; 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. 
CaroxiaA, Cantraine, 1835. 
Dedicated to Prince Charles Bonaparte. 
Syn.—Hemiplacuna, G. Sowerby. 
Disir.—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. 
Famitry PLACUNIDA. 
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. 
Puacuna, Solander. 
EKtym.—Plakous, a thin cake. Window-shell. 
Disir.—A sp. Scinde, North Australia, China. P. orbicularis, 
Retz. (exxxiii, 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 Placunz are very closely allied to Anomia; and many 
intermediate forms may be traced. The shell of each consists 
