234 AMERICAN JOURNAL 



DESCRIPTION OP A NEW GENUS OP UNIONID^. 



BY T. A. CONKAD. 



ARCONAIA, Conrad. 



Elongated, bent or bow-shaped; hinge with two distant, 

 oblique, robust, short cardinal teeth, finely rugoso-striate ; late- 

 ral teeth elongated, minutely rugoso-striate. 



Akconaia lanceolata. 



This shell has been well figured and described by Lea, 

 under the names of Triqiietra contorta and lanceolata, the latter 

 name having priority. The genus Triquetra, Klein, {Hyria, 

 Lam.,) is a very dissimilar shell exteriorly from the former, 

 and is peculiar to South America, whilst the other is limited 

 to China. But, however dissimilar the exterior of the two 

 shells, the hinge is more so. Instead of the long, compressed 

 cardinal teeth of Triquetra^ single in one valve, double in the 

 other, we find in Arconaia two comparatively short, robust 

 teeth, the posterior tooth of the left valve broad and promi- 

 nent, where in Triquetra there are numerous small, close-set, 

 radiating, compressed teeth, giving a widely different aspect 

 to the hinge when compared with Arconaia. Another differ- 

 ence may be remarked in the posterior muscular impression, 

 which is remote from the posterior margin, and of an oblong- 

 oval shape, the long diameter nearly parallel with the shell's 

 length. In Triquetra it is submarginal, large, and long in the 

 direction of the shell's height. 



