REPTILIAN AGE. JURASSIC PERIOD. 



73 



Several authors include as subgenera under this group, Oryphcea, Lamarck, and 

 Exogyra, Say ; and it cannot be denied that Ave are sometimes at a loss to determine 

 to which of these genera a given species should be referred. Indeed, in some 

 cases, we find different individuals of the same species presenting various gradations 

 between the true Oysters and Gryphseas. Whether founded in nature or not, however, 

 these groups are convenient, and can generally be distinguished without difficulty. 



The genus Ostrea appears to date back as far as the Carboniferous epoch ; at any 

 rate. Prof. Koninck has described one species (0. nohiltssima) from rocks of that 

 age in Belgium. Murchison, Verneuil & Keyserling have also referred to this 

 genus another species ( 0. matercula) from the Permian rocks of Russia. As these, 

 however, are, so far as our knowledge extends, the only evidences we have of the 

 existence of this genus previous to the beginning of the Triassic period, it would 

 seem to have been very sparingly represented, even during the deposition of the 

 latest of the Palaeozoic rocks. 



Oysters are not uncommon in the Triassic deposits, and the number of species 

 increases as we ascend through the later formations. The genus is also well repre- 

 sented, as every one knows, and probably attains its greatest development in the 

 bays, estuaries, &c. of the present epoch. 



Ostrea En§^elmanni. 



Ostrea Engdmanni, Meek, Proceed. Acad. Nat. Sci. Pliila. July, 18G0, 311. 



This species is only known from imperfect detached valves, all of which are much compressed, thin, and present 

 a more or less irregular subovate outline. The heak is truncated, and provided with a broad but short area. The 

 surface is ornamented by from seven to about fifteen irregular, moderately distinct, rather rounded, radiating plica- 

 tions, not usually extending more than about half way from the free margins towards the umbo. Lines of growth 

 regular, rather faint, and not imbricating. Internal margins not crenate near the hinge. Muscular scar rather 

 large, oval, and well defined. 



Length of largest specimen, 3.50 inches ; breadth, 3 inches. 



A. Outside under'.' valve. 

 10 September, 1864. 



Ostrea Engelmanni. 



B. Inside of another specimen. 



