130 Couthouy's Monograph on 
OssEnvaTioNs. The genus THracta, instituted 
several years since, by Dr. W. E. Leach, has been 
Siem adopted by all subsequent Zóologists. 
tds by its characters, closely allied to the Mya, 
Anatine, and Corbule, the type having been in- 
cluded, by previous British authors, among the first 
of these genera, and. by Lamarck among the second, 
under the appelle NÉ A. mydlis. M. Deshayes 
t € pof ‘Lamarck’s Anatine, a 
singular. e siculum, supported in its place 
between the cardinal eallos "S. by a portion of the 
internal ligament, wag ed th eby to examine more 
closely than had previously been done, the structure of 
analogous shells. 'The result of his researches was, the 
establishment of a family denominated Osteodesmacea, 
embracing Ist, Anatina, Lam., restricted to those 
species having a tricuspidate ossiculum, attached to the 
anterior side of the ligamentary supports: 9d, PERI- 
PLOMA, Schumacher, in which the ossiculum is subtrian- 
gular, and placed between the supports and the dorsal 
edge: 3d, Turacta, Leach, with this appendage, where 
it exists, cylindrical. and semicircular, situated at the 
posterior extremity of the internal ligament: and 4th, 
OSTEODESMA, Deshayes, having it in the form of a 
small, quadrangular plate, resting upon the internal face 
of the supports the elves. 
Although the three las named genera have been so 
recently established, 
exists, both in respect to their generic and specific char- 
acters. In the Thraciæ, this is in a measure owing to 
the type of the genus, Mya declivis, Pennant [Anatina 
myális, Lam.,] having been confounded by some of the 
ae 
wf * 
