30 
gated ovate form. In the same waters a remarkable 
variety of ochraceus is not uncommon. 
UNIO ARCTATUS. 
Plate V .— Fig . 4. 
DESCRIPTION. 
Shell elongated, compressed; slightly contracted 
from beaks to base; basal margin slightly contracted; 
anterior side short, narrowed somewhat and round- 
ed on the margin; posterior side elongated, truncated 
at the end; subemarginate above; ligament margin 
elongated, parallel with the base; epidermis blackish; 
cardinal tooth in the left valve widely bifid, not very 
prominent; within bluish white. 
Unto arctatus, nob. Silli man’s Journal, vol. xxv. 
p. 340, pi. i. fig. 9. 
OBSERVATIONS. 
This shell has a resemblance in outline to Alasmo- 
donta margaritifera. It approaches a variety of U. 
purpureus of the Delaware river, but is more elon- 
gated, and very distinct. Of many specimens which 
I found, one only was purple in the, nacre. It is 
partial to the rocky shores of the Alabama, but is 
scarcely ever seen on the bars. It inhabits also the 
Black Warrior river. This shell resembles so much 
the variety of purpureus, mentioned above, and 
which is probably U. coarctata of Lamarck, that I 
am inclined to believe several species of the Schuyl- 
kill, Delaware and other streams have been con- 
