164 JURASSIC PEIUOD. 



irregularly concave ; posterior margin rounded ; hinge-line of moderate 

 length. Right valve nearly flat or only very slightly convex ; beak short ; 

 area narrow ; ligamental groove small, shallow ; muscular scar compara- 

 tively large, subcircular, situated nearly midlength of the valve, and nearer 

 to the dorsal than to the ventral margin ; internal face of the margins appar- 

 ently not crenulated in any part. Left valve more capacious than the 

 right ; area small ; ligamental groove small ; an incipient alate expansion of 

 the postero-dorsal portion is observable upon this valve, but not upon the 

 other. Surface marked by the ordinary lines of growth, and also by a few 

 very faint indications of radiating plications. 



Length, twenty millimeters ; breadth, fouiieen millimeters. 



The collections contain a few examples, of both right and left valves, 

 in a good state of preservation, and they seem to represent a tolerably well- 

 defined species of true Ostrea. The moderately large scar of attachment 

 upon the beak of the left valve of our shell has somewhat distorted it ; but 

 there is no appearance of a curvature of the beak, such as is characteristic 

 of the genus Gryphea. 



The presence also of incipient plications upon the left valve, and of a 

 ligamental groove, like that of Ostrea, upon its area, is also against the sup- 

 position that the shell belongs to the genus Ghjphea rather than to Ostrea. 



This species differs from the young of 0. Engelmanni Meek and Hay- 

 den in being proportionally much longer and otherwise of different outline ; 

 the young of that species being broader than long, while oiu"s is longer 

 than broad, and has also the aspect of maturity. 



Position and locality. — Strata of the Jurassic period ; two miles south 

 of Dirty Devil River, Utah. 



Family PECTINIDiE. 



Genus OAMPTONEOTES Meek (Agassiz), 1864. 



Camptonectes stygius Wbite. 



Plate XIII, fig. 2 a, h, and c. 



Camptonectes stygius Wbite, 1874, Expl. & Surv. west lOOtli Merid., Prelim. Rep. Invert. 

 Foss., 23. 



Shell of moderate size, thin, lenticular; length of the hinge-line a little 



more than half the hei"'ht of the shell from l)asal marf''in to beak ; liin^fe- 



