36 



gated ovate form. In the same waters a remarkable 

 variety of ochraceits is not uncommon. 



UNIO ARCTATUS. 



Plate Y.—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. 



Unio arctatus, nob. Silliman's Journal, vol. xxv. 

 p. 340, pi. i. fig. 9. 



OBSERVA'JMONS. 



This shell has a resemblance in outline to Alasino- 

 flonta margaritifera. It approaches a variety of U. 

 purjnireus 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 Sdmyl- 

 kilL Delaware and other streams have been con- 



