MEXICAN BOUNDARY LINE. 155 



dichotoraoiis, sometimes squamose ; apex lateral, with about two volutions ; muscular impres- 

 sion profound ; upper valve with numerous elevated, concentric, squamose lamin;B. 



The Texan specimens agree in every respect with those of New Jersey and Alabama, and 

 present the same varieties ; some with, ar>d others without, ribs, and every intermediate grada- 

 tion. 



Locality. — Jacun, three miles below Laredo. 



EXOGYRA FRAGOSA. 



Plate VIII, Figure 2, a, b. 

 Exogtjrafrayosa, Con. Proc. Acad. Nat. Sci., vol. VII, p. 261). 



Orbicular ; lower valve ventricose posteriorly, flattened anteriorly ; ribbed ; ribs large and 

 prominent, broad, irregular, some of them bifurcated, crossed by robust lamellar lines of 

 growth ; umbo small, flattened, very rough and strongly ribbed ; inner margin rugosely 

 striated; upper valve plano-convex, with a very uneven, subgranulated, rugose surface, and 

 laminated towards the posterior and inferior margins ; apex marginal. 



A beautiful shell, differing from E. ponderosa or costata in having a much smaller umbo, 

 wider ribs, and more rotund outline. The margins within are finely striated and anteriorly 

 granulated. The upper valve is rugose-granulate interiorly. The figure does not well repre- 

 sent the elevation and inequality of the ribs. 



Locality. — Between El Paso and Frontera. 



GKYPH^A, Lam. 

 GRYPH^A PITCHERI. 



Plate VII, Fig. 3 ; and Plate X, Fig. 2, a, b. 



Gryphcea Pitcheri, Morton, Synopsis, p. 55^ pi. XV, fig. 9. 



Gryphcca dilatata, var. Tucumcarii, Marcou, Bui. de la Soc. Geol. de France, vol. XII, (May, 



1855,) pi. XXI, fig. 3. 



Ovate, gibbous, somewhat regular ; inferior valve inflated, arcuate, lobed ; lines of growth 

 subimbricate ; umbo large, prominent, subcompressed, incurved ; smaller valve thick, with 

 faint, impressed, radiating lines, compressed or laterally flattened above on the anterior side, 

 slightly concave in the middle ; surface concentrically imbricate-striate. 



This widely-spread species occurs in all the localities in two distinct forms : one resembling 

 G. vesicularis, and which is the type of the species as figured and described by Morton ; and 

 the other, truncated anteriorly, with a narrow, elongated, boat-sliaped umbo, var. 7iavia, pi. 

 VII, fig. 3, c, d. The upper valve of the typical form is represented in pi. X, fig. 2, a, h. 

 Roemer has given excellent figures of the var. navia. 



Localities. — Leon Springs, Texas ; plains of the Kiamesha, Arkansas ; New Braunfels, 

 Texas ; Fort Washita and Cross Timbers, Texas. 



OSTREA, Linn. 

 OSTREA SUBSPATULATA. 



Plate X, Figure 3, a, b. 

 Ostrea subspatulata, Lyell and Sowerby, Jour. Geol. Soc. London, vol. I, p. 61, (figured.) 

 Obovate; somewhat trapeziform, generally thick ; higher than wide, narrower at the dorsal 



