290 FOSSIL OSTKEID^ OF NORTH AMERICA. 



described and figured in Eeports of United States Explorations and 

 Surveys West of the 100th Meridian, Vol. IV, p. 163, Plate XIII, Pigs, 

 3, a, b, c, d. 



This shell is sometimes found associated with Gryphwa calceola, var. 

 nebrascensis; and some of the more capacious examples so far approach 

 that species in form as to suggest the possibility that 0. strigilecula 

 may really be a variety of the Gryphaia, with which it is sometimes 

 found associated. 



Ostrea (Alectryonia) procumbens White. 



(Plate XXXV, Figs. 6, 7, 8.) 



Only a few examples of this species are known, and these are all im- 

 perfect. The best of them are here figured for the first time. They 

 were discovered in Northwestern Colorado, and described in Powell's 

 Eeport on the Geology of the Uinta Mountains, page 93. 



Genus Gkyph^a Lamarck. 



Oryplicea calceola Quenstedt, var. nebrascensis Meek & Hayden. 

 (Plate XXXV, Figs. 1, 2, 3, 4, 5.) 



This American representative of the European Gryphcea calceola is not 

 abundant in our Jurassic strata, but it has been found at a considerable 

 number of localities in the great Eocky Mountain region. As already 

 remarked in connection with Ostrea strigilecula, the typical forms of this 

 species, although they have all the characteristics of Gryphwa, are found 

 associated with intermediate and transitional forms that can, with pro- 

 priety, hardly be separated from the Ostrea. The species in question is 

 fully described by Meek & Hayden in Paleontology of the Upper Mis- 

 souri, pages 74-76. Five wood-cut figures of it are given there, which . 

 are rejiroduced on Plate XXXV. 



The geographical distribution of this form is considerable, examples 

 of it having been found at distant localities in Wyoming and Idaho. 

 The first discovered American specimens, as the name implies, were 

 found in what was then a part of the great Territory of Nebraska ; but 

 it is not likely to be found within the limits of the present state of Ne- 

 braska. Although specific limitation among the Ostreidte is ofteu so 

 difficult to determine, I think it would not be unreasonable to regard 

 this form as fully distinct from the European one of Quenstedt. 



CRETACEOUS. 



The difficulty of discriminating and defining species, even among the 

 living Ostreidte, has already been referred to, and this difiiculty is far 

 greater in the case of the fossil forms. This fact will be obvious to any 

 one who scans the following annotated list, and the accompanying iJlus- 



