200 



incurved ; surface marked with fine concentric lines of growth and 

 wrinkles, which are most distinct on the upper part of the shell. 



Length of full-grown shell, 3^ inches ; height, 2i ; thickness 20 

 lines. 



This species is closely allied to P. jyiicata, D'Orbigny, from which 

 it is distinguished by its larger size and wider and more regularly 

 rounded buccal margin. 



Found in Septaria? of the age of the Ripley Group (Cretaceous) 

 at Chatfield Point, Navarro County. 



Genus Ostkea. 



O. OwENANA, (n. sp.) Shell rather large, a little oblique, ovate, nar- 

 row near the beak and expanding towards the front, which is strongly 

 rounded, lateral edges very gently convex. Superior valve moder- 

 ately thick, convex, rather elevated, greatest convexity at the middle 

 and near the front ; back obtusely rounded and more or less turned 

 to one side ; hinge area very large, triangular and deeply excavated 

 in the middle ; inferior ? valve very thick in the umbonial region, 

 and gradually diminishing in thickness to the front, which is strongly 

 curved downwards ; exterior flattened convex in the umbonial region 

 and with a broad shallow depression extending from near the middle 

 to the point; hinge area large, subquadrate, wider than long, with 

 sides subparallel and its inner margin strongly arched in the middle 

 and concave on either side ; surface of both valves marked with dis- 

 tant, concentric folds, and imbricating lamella3 of growth. 



This species is tolerably uniform in its characters. The lower 

 valve is several times thicker than the upper, and viewed in profile 

 presents a distinct sigmoidal outline. The hinge areas are marked 

 with longitudinal strise, crossed with flexuous transverse strias. 



The 0. Oicenana is related to 0. Leymerii, Deshayes, but differs 

 from that species in having the superior valve much more convex, 

 while the inferior valve is relatively much thicker. 



Length, 4J inches ; greatest width, 3 inches ; greatest thickness of 

 inferior valve, I5 inches. 



Ripley Group, Chatfield Point, Navarro County. 



Dedicated to Professor Richard Owen, -of the Geological Survey of 

 Indiana. 



OsTREA Lyoni, (n. sp.) Shell variable, oval oblong, or suborbi- 

 cular, oblique, length usually a little greater than the width, front 

 and sides rounded ; valves thin, unequal ; superior valve shorter and 

 less convex than the inferior, moderately convex in the umbonial 

 region, and flattened towards the front ; inferior valve usually rather 

 strongly convex, but variable in this respect, interior deeply excava- 

 ted ; beak acute, straight or turned to one side ; surface marked with 



