MEXICAN BOUNDAKY LINE. 157 



There are only two specimens of this species ; the outer surface of the lower valve is abraded, 

 and the characters obliterated ; the upper valve in one is much thickened on the margins, which 

 are crenulated within on the upper part. 



Locality. — Jacun, three miles below Laredo. 



OSTREA CORTEX. 



Plate XI, Figuee 4, a, d. 



Elongated, pointed towards the apex ; inierior valve ventricoso, very thick, with very promi- 

 nent, concentric, imbricated laminae ; cardinal fosset long and profound, somewhat curved, with 

 a rounded ridge on each side. 



A remarkable species, with a rough bark-like exterior ; the upper valve is somewhat ventricose 

 and marked like the opposite exteriorly. 



Locality. — Dry creek, Mexico. 



OSTREA MULTILIRATA. 



Plate XII, Figure 1^ a, d. 



Sub-triangular, thick and ponderous, somewhat curved ; both valves flattened, irregularly 

 undulated concentrically, and having numerous radiating, interrupted folds ; umbo flat, very 

 thick ; cardinal fosset long and somewhat curved, deep, cavity very shallow. 



This is a remarkable species, very variable in form. I know of no cretaceous species like this 

 or the preceding, and as no other fossil was obtained with these, their geological age is uncertan ; 

 possibly they may belong to strata of earlier date than the cretaceous rocks of Texas. 



Locality. — Dry creek, Mexico. 



UNIVALVES. 



NATICA, Lam. 

 NATICA TEXANA. 



Plate XIII, Figure 1, a, b. 



Suboval ; volutions, 5 ; rounded ; spire prominent. 

 Locality. — Between Rio San Pedro and Rio Puercos. 



NATICA COLLINA. 



Plate XIII, Figure 2, a, b. 



This specimen may be the young of the former species. The cast is distorted and imperfect. 

 Locality. — Same as preceding. 



ROSTELLARIA? Lam. 



ROSTELLARIA? COLLINA. 



Plate XIII, Figure 3, a, b. 

 One or two imperfect casts of this fossil occur with the preceding Natica. 



