the Family Osteodesmacea. 151 
supports short and subtriangular, showing a slight im- 
pression on their inner surface, as if an ossiculum might 
once have been attached to the internal ligament, which 
is thick and strong ; the external ligament is small and 
slightly prominent; posterior dorsal area broad, and 
strongly marked, extending. from the beaks nearly to 
the extremity. The interior is nearly pure white, not 
glossy, though exhibiting a slightly nacred appear+ 
ance; anterior muscular impression elongated, simple, 
club-shaped, posterior one small and very regularly 
rounded; palleal impression un deeply indented pos- 
teriorly. 
Length one nd two-twentieths, height fourteen. twen- 
tieths, diameter eight twentieths of an inch. 
Inhabits the shores of Brazil. — 
In my — and that of the Boston Soc. af Nat 
History. | 
Ozservations. ‘This species, not heretofore de- 
scribed, was obtained from a seaman who assured me 
that he procured it containing the animal, on the coast 
of Brazil, not far from Rio de Janeiro. It has a close 
affinity to T. corbuloides, Deshayes, in its general as- 
pect and the rough feeling like shagreen, so conspicu- 
ous in that shell; but differs externally in being much 
less inequivalve, destitute of the prominent ridge in 
the centre of the valves, and in having only a single 
obtuse ridge on the truncated extremity instead of two; 
internally, it differs in the structure of the ligamentary 
apophysis, which is much shorter, but at the same time 
broader, and of a more triangular shape than in T. 
corbuloides. , The anterior muscular impression also is 
