WHITE.) JURASSIC OSTREID^. 289 



JURASSIC. 



It has already been stated that no examples of the Ostreidae have 

 been found in any of the Triassic strata of North America. This defi- 

 cieucy makes a great hiatus in the geological history of the family, 

 between the Lower Carboniferous and Jurassic periods. It is probable, 

 or even practically certain, that members of the oyster family existed 

 in North America during the Triassic period, but no direct evidence of 

 it has yet been obtained. One reason at least for the absence of such 

 evidence is the great paucity of organic remains of all kinds in the 

 Triassic strata of this continent. 



The slight extent to which the Ostreidae are represented in the 

 Jurassic strata of North America, as compared with the Cretaceous 

 strata which overlie them, is perhaps largely due to a similar cause. 

 That is, the molluscan fauna of the Jurassic period is only feebly repre- 

 sented in North American strata, compared with like faunae of other 

 periods, and of the same period in other parts of the world. Still, con- 

 sidering the geological history of the oyster family as a whole, as it is 

 now known, the lack of an abundance of its remains in the American 

 Jurassic strata is probably due largely to the fact that the family had 

 not yet reached its full development. 



Four species only of the Ostreidae have been found in North American 

 Jurassic strata, but the genus Gryplima appears among them, besides 

 typical Ostrea and the subgenus Alectryonia. 



Genus Osteea Linnaeus. 

 Ostrea engelmanni Meek. 



(Plate XXXIV, Figs. 3, 4.) 



This species was originally published by Mr. Meek in the Proceedings 

 of the Academy of Natural Sciences of Philadelphia for 1860, page 311. 

 It was afterwards republished with wood-cut illustrations in Paleontol- 

 ogy of the Upper Missouri, pages 72-74. This is a well-marked species 

 of typical Ostrea ; but it is rare, only a few examples of it, mostly imper- 

 fect, having ever been discovered. These were found in what is now 

 the eastern portion of Wyoming Territory, but which was a portion of 

 Nebraska Territory at the time the species was first discovered, 



Ostrea strigilecula White. 



(Plate XXXV, Figs. 9, 10, 11.) 



At almost all localities in Wyoming, Colorado, Utah, and Idaho where 

 the Jurassic rocks are found to be fossiliferous, the shells of a small oys- 

 ter are to be found. They are usually imperfect, both by fracture and 

 also by corrosion or wave-attrition. The best examples I have seen are 

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