ARCA. 



TESTA sequivalvis vel subaequivalvis, inaequi- 

 lateralis, trapeziformls, plus minus ve ventricosa; 

 umbonihus plus minusve distantibus area liga- 

 menti separatis; caniiwe lineari, recto, dentibus 

 minimis, plurimis instrueto ; impressionibus 

 muscularibus duabus, lateralibus, distantibus. 

 Lig amentum externum. 



It is incumbent on us, in treating of this Genus, to re- 

 move erroneous impressions which may have been adopted 

 in consequence of what we have advanced under the 

 article Cucullaea. We have there asserted that the Cu- 

 cuUaeae differ from the Arcsein two circumstances, namely 

 the inequality of the valves and the want of a byssus ; 

 and we have advanced a doubt as to the propriety of ar- 

 ranging the Cucullaeae with the Arcaceae. That we should 

 now entertain a different opinion must not be regarded as 

 surprising, since we have had many opportunities of ob- 

 taining information which we did not then possess, and 

 we have become acquainted with many species of the ex- 

 istence of which we did not then know. The inequality 

 of the valves is indeed mentioned by Lamarck as charac- 

 teristic of some species of Area, and we now know several 

 species in which that peculiarity is remarkable; with re- 

 spect to a byssus, there is no doubt that the Area Noae 

 and some others are naturally affixed by a very strong 

 tendinous subtance in the same manner as the Mytili are 

 affixed by their byssus, but we have great reason for 

 doubting the existence of such an appendage in many 

 other species, for in A. Noae and such other species as we 



