GENUS THRACIA. 3 
from behind, forwards. ‘The mouth is small, transverse, oval ; 
the lips are simple, and the two pair of labial appendages are 
straight and elongated. 
The retracting muscle of the trachea of the right side is, of 
an oval form, and is continued upon the abdomen by becoming 
thin and uniting with the mantle ; that of the left side is of an 
irregular, oval form, and adheres by its internal surface to a 
membranous, elastic fold which arises from the internal open- 
ing of the large trachea, enlarging as it continues, and termi- 
nates by a semicircle which exceeds by many lines the retrac- 
tor muscle of that trachea; there, it joins the mantle. This 
apparatus, so singular and so remarkable, does not exist in any 
other bivalve mollusk at present known. It is supposed that 
the animal, by contracting it, employs it to throw out the 
water in the cavity of the trachea with more force. 
Leach first established this genus; several species were al- 
ready known, but were placed by some authors among the Mya. 
Lately, this genus has been better observed, and Deshayes, 
considering it as a type, has established many others, which he 
has united in a single family, under the name of Ostéodéme. 
The shells which belong to this genus so much resemble the 
My, the ANATIN® and the PeripLom#, that it is very difficult 
not to admit it into the family Myaria of Lamarck, for it ap- 
pears to make the step from the Mactracee to this last family. 
They have also much resemblance by their exterior form, which 
is Inequivalve, to the Corput#. ‘Their shell is generally quite 
thin; the valves are united together by an internal ligament 
placed in the spoon-shaped cavities, where these exist; the liga- 
ment extends slightly into the corselet; the valves gape only at 
the posterior side. The two muscular impressions which they 
present interiorly, are unequal, the anterior is elongated and nar- 
row towards the middle, and the posterior is rounded ; they are 
united by a palleal impression pretty deeply notched posteriorly. 
Deshayes has given such accurate descriptions of several 
species of this genus, that we prefer to borrow from him, adding 
only some new observations. 
