REPTILIAN AGE. JURASSIC PERIOD 





Eh veral authors include as suhgenera under this group, f /////-/</. Lamarck, and 

 /.'.'<;/////, Say; and it cannot In- denied that \\eare sometimes at a loss to determine 

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

 cases, we tind diti'ereiit individuals of the same species presenting various gradations 

 l)etween the true Ojsters and (inpluras. Whether founded in nature or not, however. 

 these Croups are convenient, and can generally lie distinguished without diflicnlty. 



The genus 0*irt appears to date back as fur as the Carboniferous epoch ; at any 

 rate. Prof. Konim k lias described one species (O. ////(7/Ww) from rocks of that 

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

 genus another species (0. <//>;//</) from the Permian rocks of Russia. As these, 

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

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

 seem to have been \er\ 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. 



Ontrea Engclmanni. 



Oitrea Engtlmanni, MKJUC, Proceed. Acad. Nat. Sci. Phila. July, 1860, 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 beak i> 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, 3.60 inches ; breadth, 3 inches. 



A. Outside under* valve. 

 10 September. 1S64. 



Oitrea Enijrtmanni. 



B. Inside of another specimen. 



