456 CRETACEOUS PALEONTOLOGY. 



more strongly lamellose. The upper or left valve usually con- 

 cave longitudinally, flat or slightly concave transversely, the 

 dorsal margin rounded, its cardinal regions not produced into 

 auricular appendages, the beak curved anteriorly ; surface marked 

 by distinct and somewhat regular, elevated concentric ridges 

 whose summits are produced as free lamellae. Ligamental areas 

 small or nearly obsolete. 



Remarks. This species is an abundant and long-ranging form 

 in the New Jersey Cretaceous beds, and it is also known in the 

 Eocene of the Atlantic coastal plane. It is an exceedingly vari- 

 able shell, especially the lower or right valve, which is attached, 

 the upper or left valve being much more constant in its char- 

 acters. The specimens from the different horizons where it 

 occurs all seem to be much the same, with no marked differences, 

 such as have been noted in the case of members of the genera 

 Gryphaea and Exogyra. 



Formation and locality. Marshalltown clay-marl, near 

 Swedesboro (177, 180) ; Navesink marl, near Red Bank (120), 

 Marlboro (131), Crosswicks Creek (149, 147*, 195), near 

 Jacobstown (150), near Mount Laurel (166), Mullica Hill 

 (169) ; Red Bank sand, Red Bank (123) ; Hornerstown marl, 

 near Woodstown (181); Vincentown limesand, near Farming- 

 dale (134), New Egypt (143), Vincentown (154), near Hurff- 

 ville (171, 170), near Alloway (196). 



Geographic distribution. New Jersey, Mississippi. 



Genus EXOGYRA Say. 



. ". i 

 Exogyra costata Say. 



Plate XLVIL, Fig. i. 



1820. Exogyra costata Say, Am. Jour. Sci. ist ser., vol. 2, p. 43. 

 1828. Exogyra costata Mort., Jour. Acad. Nat. Sci. Phil., ist 



ser., vol. 6, p. 85, pi. .6, figs. 1-4. 

 1830. Exogyra costata Mort., Am. Jour. Sci., ist ser., vol. 17, 



p. 284. 

 1834. Exogyra costata Mort. Synop. Org. Rem. Cret. Gr. U. 



$., P- 55. Pi- 6 , % i-4- 



