12 UNITED STxVTES GEOLOGICAL SURVEY OF THE TERRITORIES. 



On tlie other hand, Exogyra differs in having the beaks of both valves 

 strongly curved laterally (to the. left), and often distinctly spiral, with the 

 ligament-furrow narrow, following the curvature of the beak of the under 

 valve, and frequently but little defined in the upper valve. 



The genus Ostrea seems to date back to the Carboniferous period ; 

 O. nobilissima, de Koninck, being apparently a well-defined oyster from the 

 Lower Carboniferous of Belgium. Professor Winched has also described a 

 small oyster (O. patercula), said to have been found in the yellow, fine, arena- 

 ceous beds at Burlington, Iowa, equivalent to the Waverly group of the 

 Ohio Lower Carboniferous. De Verneuil has also described a small shell, 

 under the name O. matercula, from the Permian of Russia ; but it has rather 

 more the aspect of a GrypJuea than that of a true oyster. 



It is remarkable that these older species seem to have been represented 

 by so few individuals, oidy two specimens of the Belgian species being known 

 to Professor de Koninck; and but a single specimen of O. patercula was 

 found, while the more modern oysters seem always to have been gregarious. 



The shells of oysters are not uncommon in the Triassic rocks ; and the 

 number of species increases as we ascend to later formations. The group 

 Alectryonia is most numerously represented in the Mesozoic formations, but 

 diminishes in the Tertiaries ; while Gryphceostrea seems to be confined to the 

 Cretaceous and Lower Eocene. The typical oysters continued to increase 

 during subsequent epochs, and probably attain their maximum numerical 

 development at the present time ; though some of the Tertiary species far 

 exceeded in size any of the known existing oysters; O. Zongirostris, for 

 instance, having been found two feet in length. O. Georgiana, of this 

 country, also probably attained an equal size. 



Ostrea (sp. undt.) 

 Plate 2, figs. 8, a, J>. 



As I have only seen casts of the interior and impressions of the exterior 

 of this little oyster, I cannot with any degree of certainty identify it with 

 any of the described forms; and in a genus like this it would be merely add- 

 ing a name, without establishing a species on any well-defined characters, to 

 attempt to describe it as new from such material. 



As near as can be determined from the specimens yet seen, it seems to 

 be a small, rhombic-subovate species, with a moderately convex under valve, 



