62 COLORADO FORMATION AND ITS INVERTEBRATE FAUNA, [bull.106. 



mens described by Dr. White came from the same general region. 1 

 have collected it near Mancos, in southwestern Colorado, from shales 

 overlying sandstones referable to the Dakota formation and just 

 beneath a band of limestone containing Inoceramus labiatus. This 

 difference in geologic horizon would not justify the separation of these 

 fossils from G. pitcher i if they were identical in form. But while there 

 is a general resemblance, and possibly examples of the G. pitcheri of 

 Texas might be selected that could not be distinguished from some of 

 the western ones, the differences are as a rule sufficient to make them 

 easily separable. The fact that the western form occurs in later beds 

 and that all its faunal associates are different necessarily gives greater 

 importance to slight differences in form. Compared with typical ex- 

 amples of G. pitcheri it has broader and less distinct beaks, smoother 

 surface And more rounded outlines, and it never attains so large a size. 

 The specimen represented by PI. v, Figs. 3 and 4, is the largest I have 

 seen. 



Some of the smaller examples have a superficial resemblance to 

 Gryphcea vesicular is var. micella Roemer, but the latter is more alate 

 and its beak is not so much incurved. 



Of European species small examples of Ostrea proboscidea Archiac 

 and some specimens of Ostrea vesiculosa Gueranger as figured by 

 Coquand * closely resemble this species. 



Genus EXOGYRA Say. 



Exogyra suborbiculata Lamarck (sp.). 



PI. v, Fig. 6; PI. VI, Figs. 1 and 2; PI. vm, Fig. 1. 



Gryphwa suborbiculata Lamarck, 1802, Systeme des Animaux sans Vertebres, p. 398. 



Gryphites ratisbonensis Schlotheim, 18i3, Min. Tascli., vol. vn, p. 105. 



Gryphcea columba Lamarck, 1819, op. cit., vol. vi, p. 198. 



Ostrea ratisbonensis Coquand, 1869 (with full synonymy), Monog. Gen. Ostrea, Terr. 



Cr6t.,p.l21. 

 Exogyra suborbiculata Stoliczka, 1871, Cret. Pelecypoda of Southern India, p. 462. 



Shell large, comparatively thin, irregularly suborbicular or subovate 

 in outline. Lower valve very convex; umbonal region narrow and 

 prominent; beaks relatively small, distinctly coiled, more or less sepa- 

 rated from the body of the shell. Full-grown individuals have a broad 

 distinct furrow or depression extending obliquely from the umbonal 

 region to the posterobasal margin. Surface smooth, marked only by 

 lines of growth. Upper valve represented in the collection by frag- 

 ments that show that it was suborbiculate, nearly flat, with distinctly 

 coiled beak and marked by obscure concentric ridges near the periphery. 



An average sized specimen gives the following measurements: 

 Length from beak to base, 82 mm ; greatest transverse breadth, 80 mm ; 

 convexity, about 43""". 



1 Monog. Genre Ostrea, Terr. Cr6t., pis. 16 and 59. 



