"^nn^l CRETACEOUS. 307 



Exogyra wallceri White. 



(Plate LIV, Figs. 1,2.) 



This is a large, compressed form, which comes from the Cretaceous 

 strata of Texas. It was published in the Annual Eeport of the United 

 States Geological Survey of the Territories for 1877, page 278, Plate I, 

 Figs. 1, a, b. 



Exogyra winchelli White. 



(Plate LV, Figs. 6,7; Plate LVI, Figs. 1,2.) 



The form which is most nearly related to this species is the E. halio- 

 toidea of Sowerby, but it is more elougate than that shell, and the front 

 side is more abruptly elevated. It is probable that this American form 

 might be with propriety recognized as a variety of the European E. 

 haliotoidea, but I prefer at present to regard it as distinct. I have 

 recognized the last-named species among some Cretaceous fossils from 

 Brazil, and they seem to be sufficiently distinct from E. whichclU. It is 

 only in the Cretaceous strata of the Gulf States that E. icinclieUi is yet 

 known. It was published iu the Proceedings of the United States 

 National Museum, Vol. II, page 294, Plates II and III; and also iu the 

 Annual Eeport of the United States Geological Survey of the Territories 

 for 1878, page 12, Plate XIII, Figs. 1, a, b, c, d. 



LARAMIE GROUP. 



The great brackish water formation of Western North America, which 

 is known as the Laramie Group, has already been referred to. In this 

 great formation no true marine fossil remains have been found, but 

 oyster shells are not unfrequently found in its strata, and in some places 

 they are abundant. Among these no less than five species have been 

 proposed by different authors, but later collections show such gradations 

 of form that I have not been able to recognize more than two species 

 among them, and it is probable that there is really only one species in 

 the whole formation. 



These oyster remains of the Laramie Group not only belong to the 

 typical genus Ostrea, but the most abundant of the two recognized 

 species is very closely like the living Ontrea virginica. This species is 

 quite constant in its typical form, even at points more than a thousand 

 miles distant from eacli other, and the extent of its geographical distri- 

 bution seems to have been quite equal to that of the living 0. virginica. 



Osfrea glabra Meek & Ilayden. 



(Plates LVIII, LIX, LX, LXI.) 



This widely distributed species was first published under the above 

 name in the Proceedings of the Academy of Natural Sciences of Phil- 



