100 
GEOLOGICAL REPORT—THIRTY-FIFTH PARALLEL. 
particularly towards the margins, and sometimes with radiating striae in the centre. The sin¬ 
uosity and elevation in the postero-ventral margin corresponds to the depression in the opposite 
valve, and in small specimens is often not conspicuous. In young shells, the surface of the 
lower valve is not unfrequently costate-imhricate. 
This shell, in some of its varieties, resembles G. vesicularis , and the upper valve might he 
mistaken for that species ; the strongly incurved heak and the more direct sinus of the larger 
valve are distinctive characters. 
It has clearly the typical form and characters of the species, as will be seen by referring to 
Dr. Morton’s figure of a small individual. Numerous specimens from different localities have 
been examined, and the characters are sufficiently constant in all to leave no doubt of the 
specific identity. The specimens figured by Dr. Roeiner in his Kreide von Texas, as G. Pitcheri, 
may he an extreme variety of that species, but we have not yet seen specimens showing satis¬ 
factorily the gradation from one to the other. The specimens upon which Mr. Marcou pro¬ 
posed the new species, are evidently much larger and finer ones than were possessed by Dr. 
Morton when he wrote his description and gave the figure of this species. The original of his 
figure and description, however, is still preserved in the cabinet of the Philadelphia Academy 
of Natural Sciences. 
Locality. —Brought by Mr. Marcou from Pyramid Mountain, Plaza Larga, Camp 50. 
GRYPILEA PITCHERI, var. NAVIA.— Plate I, figs. 7-10. 
Grypilea Pitcheri, Poemer, Ivreide von Texas, page 73, plate IX, fig. 1. 
Shell elongate, narrow-subovate ; lower valve longitudinal, somewhat trilobate ; a distinct 
depression extending from the posterior side of the beak to the base of the shell; anterior side 
flattened or slightly depressed; middle gibbous and extremely arcuate; heak abruptly incurved, 
laterally compressed, and turned to the posterior side; hinge line short; upper valve unknown. 
Specimens of this form are very abundant, judging from the collections made in the west. 
The general aspect of the shell is that of G. Pitcheri , but it is much narrower, more elongated, 
and more trilobate than the typical forms of that species. The larger valves are usually very 
thick, and often distorted from this cause. The extreme incurvation of the heak is most con¬ 
spicuous on the exterior ; and though the thickening of the shell sometimes obliterates this 
character upon the inner side, it is, nevertheless, a characteristic feature. This form is the one 
usually referred to G. Pitcheri , and in its extreme character is well represented in the figures of 
Dr. Boeiner, cited above. Like the preceding, in its young state it is sub-costate-imbricate on 
the larger valve ; hut this character disappears with age, and the lamellose structure becomes 
very conspicuous sometimes on the ridges, rising into squamose node-like elevations. 
Locality. —False Washita, Camp 31. The same has likewise been brought from numerous 
other localities in the west. 
OSTREA, Linneus. OSTREA CONGESTA.—Plate I, fig. 11. 
Ostrea Congesta, Conrad , Nicollet’s Report, page 167. Washington, 1843. 
“ Shell elongated, upper valve flat, lower valve ventricose, irregular ; the umbo truncated by 
a mark of adhesion.” 
In the young specimens, when not distorted by pressure, the form is obliquely ovate, with a 
sharp triangular beak curving to the left, and a small well-defined area, the shell adhering by 
the entire outer surface ; the margins are abruptly inflected, so as to become vertical to the 
plane of the adhering surface and continue to grow in that direction, the inner side crenulated 
at the junction of the upper valve. In older specimens the form is distorted by the pressure of 
contiguous individuals, and the heak truncated by adhesion, and finally becomes obsolete ; the 
entire valve often assumes a somewhat semi-cylindrical or tubular form. 
The shell is extremely abundant, and in its stages of growth presents all the variable charac¬ 
ters of form observed in other species which live in closely aggregated masses. 
