CRETACEOUS FOSSILS. 171 



sinus; posterior muscular impressions very faintly mai'ked; anterior muscu- 

 lar impressions generally well defined, subovate, attenuated, and curving 

 backward above where they connect with the small scars of the pedal 

 muscles. 



Length of a medium-sized specimen, 1.33 inches; height of the same, 

 0.87 inch; convexity, 0.78 inch. 



This species varies considerably in form, as well as in surface-markings. 

 In some specimens, the whole surface is nearly smooth or only marked with 

 obscure lines of growth (fig. 2a) ; while, in others, the most gibbous part of the 

 valves and the umbones are marked with very regulai*, distinct, concentric 

 ridges and furrows (fig. 2). In still others, the ridges and furrows are exceed- 

 ingly irregular and very strongly marked (fig. 2c). There are also more or 

 less marked difierences in the elevation of the beaks, the convexity of the 

 anterior region, and the proportional length of the attenuated posterior 

 extremity. Yet all of these varieties blend together by such sHght shades of 

 difference that it seems impossible to find constant characters by which they 

 can be separated specifically. 



In first describing this species, from the few imperfect specimens orig- 

 inally brought from the Far- West, I was led to believe that there were two 

 distinct species represented among them; one nearly smooth, which I called 

 Corbiila pyriformis, and another with regular, concentric ridges, for which I 

 proposed the name C. conccntrica. The extensive series of specimens, how- 

 ever, brought in by various explorers from the same locality, soon led to 

 the conclusion that the two types really belong to one variable species. 



None of the specimens originally studied were in a condition to give 

 any clew to the nature of the hinge; but, from the fiict that they were found 

 associated with some fresh-water shells, it seemed quite probable that they 

 belonged to the brackish- water group Potamomya, or more properly Azara, 

 and for that reason I placed the name Potamomya, with a mark of doubt, 

 parenthetically between the generic and specific names. Some years later, 

 however, when large numbers of fine specimens of this shell were at hand 

 for study, I succeeded in working out the hinge, and found that it does not 



