218 
being composed of laminae whose edges rise succes- 
sively in more or less elevated scales upon the surface. 
I^ike Gryphaea it is sometimes attached only by the apex 
of the convex valve, and is free when full grown ; and 
sometimes by a large part of its surface. Some species 
are ribbed, but irregularly, like oysters. 
The mistake under which several shells that might 
better have been called Gryphaeae w 7 ere published in the 
first volume of Mineral Conchology as Chamae, seems 
to have arisen from considering the tooth in the hinge 
without taking into account the muscular impressions, 
which in Chama are two. This important tooth seems 
to have been overlooked by Say, although he mentions 
the cavity or furrow that receives it. In the sixth vo- 
lume of the Journal of the Acad, of Nat. Sciences of 
Philadelphia, Dr. Morton, when describing the Exogyra 
costata of America, has rightly referred the Chama ha- 
liotoidea, (Min. Conch, t. 25.) to the same genus, and dis- 
covers the mistake above alluded to ; but is rather hard 
upon English Conchologists, in supposing that they join 
in the blunder. 
The shells formerly published in Min. Conch, that 
should be arranged under this genus, are 
Chama haliotoidea, tab. 25.* 
recurvata 
tab. 26. 
f. 2. 
conica 
tab. 26. 
f. 3 . 
plicata 
digitata 
tab. 26. 
tab. 174. 
f. 4. 
Chama canaliculata tab. 26. is a Gryphaea. 
Thus far it appears that the genus Exogyra is bound- 
ed by distinct lines of demarcation ; but in fact it is no 
more insulated than any other artificial genus ; for Gry- 
phasa nana and G. sinuata have often the beaks laterally 
curved, and at the same time possess the characteristic 
lobe of Gryphaea, and we have seen several recent Oysters 
whose beaks strongly resemble those of Exogyra. 
* Miss Benett has kindly lent us a noble specimen of Ex. ha- 
liotoidea nearly four inches long, which we propose to figure in 
a Supplement. 
