216 NEW UNIONID/E OF THE UNITED STATES AND ARCTIC AMERICA. 



Embryonic shell triangular, large, of a brown color and altogether like that of 

 A. undulata, Say. 



Remarks. — I have two specimens from Dr. Showalter in alcohol, and Dr. Hart- 

 man has two larger specimens without the soft parts. The outline of this species 

 is very close to some of the forms of A. undulata, Say, and it also has the aspect of 

 A. pavonia, (nobis.) It has subconcentric folds on the tips of the beaks \ik.% pavaivia, 

 but they are smaller. Like undulata it has a callus on the dorsal margin on each 

 valve under the beak, and in Showalterii this callus is quite erect and compressed, 

 tooth-like, so that it seems to be the link between Margaritana and Anodonta proper. 

 It is more inflated than pavonia or undulata. From its close alliance to the latter 

 in the form of the shell, it would seem to be likely that the important organ of 

 the branchial uterus would be somewhat alike. In their position in the branchiae 

 they are entirely different, the undulata having the branchial ovisacks transverse, 

 while the Showalterii has them vertical as in A. fluviatilis. It gives me great plea- 

 sure to name this species after Dr. Showalter, who has done so much to develope 

 the molluscs of that part of Alabama in which he lives. 



