* 
Mr. Barnes on the Genera Unio and Alasmodonta. 113 
chology, which is very desirable in every science, that the 
me terms should have a uniform meaning. Having learn- 
ed i in univalves what is the mouth and what is the base of a 
shell, we apply the same terms to bivalves; but to call the 
thin, sharp, unconnected edges of a shell, the dorsum or 
back would sound very strangely. M. Lamarck has not 
not as to right and left. We call the connected part of a bi- 
valve the back and the opposite the base. 
If this is determined, there will remain another point to 
be settled. Authors have very generally agreed in calling 
that side of the beaks in which the ligament is ee niale 
ona and the opposite, ie posterior. “ But rigidly 
aking,” says Mr. Say, “we seem to be all aie in 
pe adaptation of these sclativete terms, because the latter is 
used to indicate that part of the shell van covers the 
mouth of the included animal, and which is foremost in its 
progressive movements. In order to be correct in descrip- 
tions where the animal is referred to, these terms must be 
reversed, and if in descriptions which have reference to the 
animal, certainly the principle applies to all other bivalves, 
in which the mouth is similarly situated. The mouth ought 
always to be considered as in the anterior. For this reason, 
Cuvier reverses the terms right and ek applying the for- 
r to that valve of the Uniones which has but a single 
lamelliform tooth, and which is our /eft valve.* He of 
course, reverses the anterior and posterior as now applied.’’t 
It would surely be deemed safe to follow an author so pre- 
eminent as JM. Cuvier, and this mode of viewing the shell 
is doubtless most conformable to nature; but as all other 
authors have a different view, we have ‘resolved, for the : 
present, to adopt the established usage of the term anterior - 
of posterior, and to follow M. Lamarck as to right and 
If we rightly understand the celebrated French Natural- 
ist, he is under a mistake in saying that the Uniones “keep 
* This agrees with Burrow. +Mr, Say’s MSS, 
Von. VI.—No. i» 15 
a pe 
