181 
in the above description an elevated line^ is, strictly speaking, the 
line of junction with the basal surface which is a little more elevated 
than the remaining surface of the volution, making a kind of step, 
and not in reality a simple elevated line. In my observations on 
this species, in the Journal above mentioned, I remarked that it 
very much resembles the clatliratulus, Montagu, but the lip is 
more robust and the basal portion of that part is more dilated,’’ 
PL 27 ; lower left figure. 
Amphidesma. — Shell rounded or transversely somewhat oval, 
subinequilateral : hinge with one or two cardinal teeth, and having 
a lateral tooth each side, sometimes obsolete ; a slender oblique 
fosset originating immediately under the beak and passing an- 
teriorly : ligament divided into two parts, of which the true liga- 
ment is exterior and very short, and the cartilage portion is interior, 
affixed in the oblique fosset : muscular impressions two, remote, 
not elongated : parallel line with a much dilated and profound 
sinus, 
Ohs. — The characters of this genus are very distinct and easily 
recognized from all those that have no part of the ligament inter- 
nal. Lamarck first distinguished it by the name of Donacilla^ 
which I think ought to be retained on the principle of priority, 
and that of AmpMdesma, that he afterwards applied to it, ought 
to be rejected. He has associated it, in the same small group, 
with IJngulina and Solemya, from both of which it may be known 
by its much dilated and profound sinus of the palleal line. Other 
genera of his family of Maetraceoe have divided ligaments, particu- 
larly Mactra, with which, in fact, Linne arranged the species; but 
their cardinal fosset is more direct, dilated and deltoid, whilst that 
of Amphidesma is nearly parallel with the edge, and fusiform. Lii- 
traria is entirely destitute of lateral teeth, the valves gape and 
the ligament is fixed in a deltoid cavity. Blainville unites Am- 
phidesma with Lucina and with Fimbria, Megerle, or Corhis, Cuv. 
But Corhis has lateral teeth, an exterior ligament, though deeply 
seated ; and the parallel line is entire, and the posterior muscular 
impression is elongated. 
Amphidesma transversum. — Specific character. Trans- 
versely short oval ; hinge nearly central. 
Desc. — Shell transversely short oval, nearly equilateral, com- 
pressed, a little gaping : anterior and posterior margins subequally 
