170 CRETACEOUS PERIOD. 



MOLLUSCA VERA. 



Class CONCHIFERA. 



Order MONOMYAEIA. 



Family OSTEEIDJ^:. 



Genus OSTREA Linnains, 1858. 

 Ostrea cortex Conrad. 

 Plato XV, fig. 2 a, b, aud c. 

 Ostrea cortex Conrad, 1857, U. S. & Mex. Bound. Surv., i, 157. 



Shell elongate, moderately capacious ; test of mature examples thick ; 

 infei'ior valve much more capacious than the other, narrowing towai'd the 

 apex, which is pointed and rather small ; ligamental furrow long, mod- 

 erately broad and deep, somewhat curved, its raised borders broadly 

 rounded ; upper valve corresponding with the lower, except that it is usually 

 nearly flat transversely. 



Surface of both valves marked by numerous strong, prominent, imbri- 

 cating concentric laminas, which are sometimes laciniate, giving the shell a 

 very rough appearance. 



Length of the largest example in the collection, about one decimeter ; 

 breadth, nearly five centimeters. 



Conrad's type-specimens of this species were obtained at "Dry Creek, 

 Mexico," from strata the age of which was then unknown to him, because 

 the collectors brought no associated species. Our collections, however, con- 

 tain associated species that indicate unmistakably their Cretaceous age. 

 Our examples show that the species is a very variable one; but they all 

 present a certain facies that is quite characteristic, and which is expressed 

 in the figures, both of Conrad's and this report. It is an interesting fact 

 that the test in nearly all the examples of this species has been made com- 

 pletely porous by a species of Cliona or other burrowing sponge, Avhile 

 those of the following species are almost entirely unaffected by it. Possibly, 

 however, the case of the last-named species is only a local exception. 



Position and locality. — Strata of Cretaceous age ; Colob Plateau, south- 

 west of Kanara, and also at the North Fork of Virgin River, Utah. 



