the Family Osteodesmacea. 153 
"Tungacia CONRADI. , 
Plate IV. fig. 2. 
Th. test albo-cinerescente, ovato-transversi, ventricosi, subequi- 
laterali, fragili, paullum hiante, margine sinuato, anticé rotundatà, 
posticé subtruncata, carinà obtusi, ab apice ad marginem infero- 
posticam sedente. ligamento Kit prominente, interné callo 
e napa valvà utraque inserto. 
"Thracia declivis, Conran, Amer. Mar. Conch. p. 
44, tab. 9, fig. 2. Synon. omn. exclus. 
 dnatina convéxa. Catal. Animals and Plants of Mass. 
1834, p. 25. 
Description, Shell ne a ovate, ventricose, 
very light, brittle and thin; rather faintly diaphanous by 
reason of its want of thickness, subequilateral, slightly - 
gaping at both extremities, inequivalve, the right valve 
being the more convex, its whole margin projecting con- 
siderably beyond that of the left; beaks protuberant, 
large and cordiform, inclining a little backwards, the 
summit of the right one excavated or emarginate to re- 
ceive the opposing one; incremental strie numerous 
and distinct, occasionally forming feeble concentric 
ridges; the anterior portion of the shell is regularly 
rounded, and its superior margins very thin ; the poste- 
rior extremity is rather. narrower and somewhat trun- 
cated, with an obtuse carination extending obliquely 
from the beaks to the angle of the basal and posterior 
margins ; ; between this carination and the superior and 
posterior margins, the shell is slightly compressed. ‘The 
basal margin is sinuous, curving outwardly in its central 
portion, correspondent to, the most convex part of the 
