ON SOME PLIOCENE FRESH WATER FOSSILS OF 

 CALIFORNIA. 



BY J. G. COOPER. 



Margaritana subangulata n. sp. Plate xiv, figs. 

 1-4. 



In form nearly intermediate between M. mar gar it if era 

 Linne and M. marginata Say, beaks more prominent 

 and anterior than in the former, less so than in the latter, 

 with twelve or more slight undulations as in M. undnJata 

 Say. A strong obtuse ridge extends from the beaks to 

 the posterior basal angle as in Anodonta angulata, with 

 a gentle curve, and the whole outline is very similar. 

 The hinge, however, is that of a Afar garit ana, as 

 shown in the figure. It is about one -seventh of the 

 length of shell from the anterior end. Surface smooth, 

 except near the beaks, w^ith no irregularities else- 

 where. Basal margin var3nng from slightly convex to 

 slightly arched. Few variations in size or form in the 

 specimens found, five of them nearly perfect. Length, 

 3.25 inch; height, 1.50; breadth, 0.90. The hinge, 

 worked out from above in three shells, is like that of M. 

 margaritifcra, and the shell is thicker than that of Ano- 

 donta, which led me to examine the hinge, a portion of 

 which was exposed in one specimen. Its great resem- 

 blance to A . angiilata, however, arouses the suspicion 

 that it may be the predecessor of that peculiar species, 

 having lost its teeth by migrating from running water to 

 quiet lakes, where teeth are not needed to secure the 

 valves, and the shell decreased in thickness. Dr. A. A. 

 Gould suggests such transformations as possible among 

 living species, and if proved, they would aid very much 

 in explaining the vast number of forms among our Amer- 

 ican UnionidcB. All the California Anodontas inhabit 



2D Ser., Vol. IV. May 26, 1894. 



