326 Dr. Maton’s Description of 
thin, semitransparent substance, not unlike bladder in texture, 
and soluble in nitrous acid. The young shells are attached to 
it by their epidermis, which, in fact, seems to be merely a mem- 
— branous expansion of the same substance, and to take its origin 
from it for the purpose of confining the animal during the for- 
mation of the shell. In some species, the attachment of the 
membrane is so loose, that it is thrown off very soon after the 
animal is set at liberty; but in others it remains firmly adhering 
to the calcareous matter during life. Most fluviatile shells retain 
this covering more or less entire, and it is the case with all the 
species hereunder described, in all their stages of growth. ‘The 
membrane by which the calcareous matter of the shell is se- 
creted, or deposited, is of a very different nature, and has a more 
immediate connexion with the contained animal. 
1. Mya LABIATA. 
Tas. XXIV. Fig. 1,2, 3. 
: Mya testa subovali, valvis occlusissimis, alterius margine 
l abii instar) prominente. 
Habitat in America australi, fluviatilis. 
Testa firma, transversim striata, epidermide viridi, leviore, de- 
ciduá, intus margaritaceo-polita, anterius subrostrata. Cardinis 
dens alterius valve solidus, subcochleariformis, antrorsum por- 
rectus, foveæ triangulari valve opposite insertus. Margo hujus 
(è regione cardinis) quasi truncatus, illius rotundatus, subtenuis. 
Umbones parum prominentes. 
I have not mentioned the size of Mya labiata in the above de- 
scription, not thinking myself warranted so to do, unless I had 
seen a great number of specimens. ‘Those from which the cha- 
racters were taken are all of the same size, and about 1 inch in 
Bi. length, 
