114 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



as forming a single group, itself only differing subgenerically from Unio 

 proper. 



The only essential difference observable between the shells of Marga- 

 ritana (including the above sections) and Unio, is the entire absence of well- 

 defined posterior teeth in the former. Professor Agassiz, however, observed 

 some differences in the structure of the animal, in certain species of Margari- 

 tana, and that of the typical Uniones; that is, in the former he found the gills 

 free from the mantle, at their posterior extremities, instead of being united 

 to it, as in Unio proper ; and the anal opening or region of mantle in Marga- 

 ritana not fringed, as it is at both siphonal openings in Unio. 



The group Margaritana is known to date back to the Wealden of 

 Europe ; it also occurs in the Tertiary, and, like the true Uniones, seems to 

 attain its greatest development in the existing rivers and smaller streams of 

 North America. There are two species and many varieties now living in the 

 fresh-waters of Europe. 



margaritana IVebrascensis, Meek 



Plate 1, figs. 5, «, 1), c. 



Unio (Daphia ?) Xebrascensis, Meek (1871), Haydeu's See. Ann. Rep. U. S. Geol. Snrv. of the Territories, 303. 



Shell attaining a medium or larger size, thin, cuneate-subovate, being in 

 the adult very gibbons anteriorly, and cuneate behind ; anterior side very 

 short and rounded ; posterior sloping above obliquely from the end of the 

 hinge to the posterior basal extremity, which is narrowly rounded ; basal 

 border sinuous behind the middle, and convex in front of it; cardinal margin 

 rather short and nearly straight, or slightly arched; umbones very gibbous, 

 but depressed, oblique, incurved, and placed near the anterior extremity ; 

 posterior umbonal slopes subangular from the beaks obliquely backward and 

 downward to near the middle, beyond which they are continued as broadly- 

 round ridges to the posterior basal extremity ; below and parallel to these 

 ridges, there are also, on the flanks, one or two large, oblique, irregular, 

 rounded plications or undulations, that continue on to the sinuous posterior 

 basal margin, to which they sometimes impart a distorted or waved appear- 

 ance. Surface otherwise smooth, excepting moderately distinct lines of 

 growth, which are strongly undulating in places as they cross the oblique 

 ridges or plications of the Hanks. 



