Mr. Barnes on the Genera Unio and Alasmodonta, 113 



chology, which is very desirable in every science, that the 

 same terms should have a uniform meaning. Having learn- 

 ed 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 

 ventured on so strange an expression ; but says commonly 

 the upper margin, the same that Mr. Say calls the "basal 

 edge." According to this view of the subject we should 

 agree with M. Lamarck and the Encyclopedia as to right 

 and left, but not as to hase; and with Burrow as to base, but 

 not as to right and left. We call the connected part of a bi- 

 valve the hack 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 situated, the 

 anterior, and the opposite, the posterior. " But rigidly 

 speaking," says Mr. Say, "we seem to be all wrong in 

 our adaptation of these relative terms, because the latter is 

 used to indicate that part of the shell which 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 left, applying the for- 

 mer to that valve of the Uniones which has but a single 

 lamelliform tooth, and which is our left valve.* He of 

 course, reverses the anterior and posterior as now applied."! 

 It would surely be deemed safe to follow an author so pre- 

 eminent as M. Cuvier, and this mode of viewing the shell 

 is doubtless most conformable to nature ; but as all other 

 authors have a difTerent view, we have resolved, for the 

 present, to adopt the established usage of the term anterior 

 and posterior, and to follow M. Lamarck as to right and 

 left. 



If we rightly understand the celebrated French Natural- 

 ist, he is under a mistake in saying that the Uniones "keep 



* This agrees with Burrow. t JVfr. Sav^s MSS, 



Vol. VI.— No. 1. 15 



