MEXICAN BOUNDARY LINE. 153 



CYTHEREA, Lam. 



CVTHEREA LEONENSIS. 



Plate VI, Fro. 1. 



Oblong-subovate, ventricose, very inequilateral ; posterior margin, from beak to extremity, 

 sligbtly sinuous ; extremity truncated or obtusely rounded, direct. 

 Locality. — Leon Springs, El Paso road. 



CYTHEEEA TEXANA. 



Plate VI, Fia. 2. 



Obliquely-ovate, ventricose, very inequilateral, with prominent lines of growth ; umbo large ; 

 umbonal slope subangulated ; buccal margin obtusely rounded ; base profoundly rounded ; 

 dorsal margin straight, very oblique. 



Locality. — Between El Paso and Frontera. 



PLICATULA, Lam. 



PLICATTJLA INCONGEUA. 



Plate VI, Fig. 10, a, b. 



Ovate, small, lower valve ventricose, with prominent entire ribs bifurcating from the umbo ; 

 superior valve flattened, with squamose, scarcely prominent, ribs ; interstices linear. 



Fig. 10 a represents the flat squamose valve, and fig. 10 b the opposite smooth-ribbed valve, 

 as they appear in relief on a piece of hard limestone. 



EXOGYRA, Say. 



This genus, which is related to Gryphcea, originated in the Oolitic epoch. It widely differs 

 from Ostrea, though some authors, even at the present day, include the species in that genus. 

 The fact that all the species died out before the oldest Tertiary period, favors the idea that the 

 animal was somewhat differently organized from tliat of Ostrea. 



EXOGYEA ARIETINA. 



Plate VII, Fro. 1, a-e. 



Exogyra arietina, Roemer, Kreide. von Texas, p. 68, pi. VIII, fig. 10. 



Exofjyra arietina, var. c(q:>rina,Con. Jour. Acad. Nat. Sci., Vol. II, new series, p, 273. 



Ventricose ; larger valve having the umbo spiral, or shaped like a ram's horn ; back with 

 obtuse or obsolete angles and furrows, and undulated, subimbricated lines of growth ; upper 

 valve nearly flat, with concentric lamellose lines. Very abundant. The variety caprina is 

 generally elegantly marked with distinct, prominent, radiating, interrupted, subnodulose ribs. 

 On the weathered surface of the rock they project in great perfection, and are crowded in vast 

 numbers. It is related to E. FelUcoi, Gervais. 



Locality. — Leon Sjirings. 

 20 M 



