148 GEOLOGY AND PALEONTOLOGY. 



TRIGONIA EMORYI. 



Plate III, Figure 2, a, h, c 



Trigonia Emoryi, Con. Proc. Acad. Nat. Sci., vol. 



■ Inequilateral, obliquely truncated posteriorly, alated ; obliquely ribbed ; ribs about 34, narrow, 

 prominent, compressed, or laterally abrupt, nodulose, diverging near the dorsal margin ; ribs 

 posteriorly, about the umbonal slope, composed of series of small nodules. 



The form of this species approaches nearest to T. crenulata, but it is wider across the middle 

 between the buccal and umbonal slope. The form of ribs is most nearly like those of T. scdbra, 

 but that shell has a wider anal area, and the dorsal depression is much smaller, being of a more 

 ovate cuneate form. 



Locality. — Between El Paso and Frontera, 



TRIGONIA TEXANA. 

 Plate III, Figure 3, a, h, c. 



Trigonal cast of a large species, which is profoundly ventricose, truncated and direct on the 

 ventral end, summits profoundly elevated ; shell unknown. 

 Locality. — Leon Springs. 



MACTRA, Lin. Lam. 



MACTRA TEXANA. 



Plate IV, Figure 1, a,h. 

 Ilactra Texana, Con. Proc. Acad. Nat. Sci., vol. VII, p. 269. 



Triangular, ventricose, subequilateral, buccal end subangulated and slightly produced, much 

 above the line of the base, which is regularly and profoundly curved, anal margin obliquely 

 truncated, extremity angulated ; buccal margin straight and very oblique ; summit prominent. 



This species is known from casts, and may prove to belong to the genus ScMzodesma, Gray. 



Locality. — Prairie between Laredo and Rio G-rande city. 



CUCULL^A, Lam. 



CUCULL^A TERMINALIS. 



Plate IV, Figure 2, a, b. 



Ovate-triangular, ventricose ; buccal margin in the cast almost direct ; anal side produced, 

 cuneiform, extremity angulated ; basal margin straight ; summit profoundly prominent. 



I have not seen the shell of this species. Comparing the casts with those of C. vulgaris, 



Morton, the summits are found to be much more prominent and more distant, as well as more 



nearly terminal. 



Locality. — 



ARCA, Lin. 



ARCA SUBELONGATA. 



Plate VI, Figure 3, a, 6. 

 Trapezoidal ; anterior end regularly and rather acutely rounded ; hinge and basal margins 

 parallel • posterior extremity obliquely truncated. 

 Locality. — Between El Paso and Frontera. 



