6 PALEONTOLOGY OF THE UPPER MISSOURI. 



near two hundred individuals, and yet they occur in all parts of the mass, so that 

 every new surface exposed in splitting it, is seen to be covered with them. They 

 are not crushed or distorted in the slightest degree, and are aU casts, the shell 

 itself being in no instance preserved. 



The species is allied to Theca? trianffidcu-ls. Hall (Palrcont. N. Y. Tol. I, p. 313, 

 PL Ixxxvii, Fig. 1 a, h, c, d), but never attained near so large a size, and is more 

 rounded on the ventral side. It is probably more nearly related to T. jprimordialis, 

 HaU (An. Eept. Geol. Survey Wisconsin, 1861 1 p. 80), but never attains more than 

 about half the linear dimensions of that species, and we have seen no traces of the 

 arching midulations on its flat side, mentioned in the description of T. primordialis, 

 though some of the impressions in the matrix show very faint indications of fine 

 arching transverse striae on this side. In size and form it agrees quite closely with 

 Salterella obtusa of Billings (New. Sp. Low. Sil. Foss., p. 18, Montreal), but it seems 

 to be a thinner shell, and shows no evidences of being composed of successive 

 layers, nor is it provided with the sharp annulations seen in the type of that genus. 



Locality and j^)osi(iun. Near the head of Powder River, in Big Horn Mountain, 

 Idaho Territory. From the Potsdam or Primordial Sandstone. (No. 1181.) 



