FOSSIL SHELLS AND TEETH OF SHARKS. 
171 
Specimens of the genus Natica are the most abundant. They occur of all sizes intermediate 
between those represented, figure 57 being the largest one that was obtained. Natica geniculata 
is described by Mr. Conrad from the drawing of the cast of the interior of the shell, Plate VII, 
fig. 67. This cast, when among the others, appeared to me to be like the species represented in 
figures 64 and 65. Casts of this form were numerous, and the exterior of the shell was shown 
by the impressions of the outside to have been rugose and tuberculated. A fragment of one of 
these impressions of the exterior is represented in fig. 66. 
Of the genera Pleurotoma and Turritella (PI. VII, figs; 69 and 71, PI. VIII, figs. 73 and 74,) 
but three or four specimens were found. The casts of the interior were very perfect, even to the 
end of the spire. 
The bivalves were very abundant, and generally of large size. Forms similar to fig. 79, 
Plate VIII, (Dosinia ?) and 81 and 82, (Venus?) were most frequently found. Tellina (PL VIII, 
fig 75) and others, perhaps the same species, were very numerous. The forms of both the inner 
and outer portions of all these bivalves were well preserved, but the valves were all separated. 
One specimen only of the form shown by fig. 78, PL VIII, was observed. The figures repre¬ 
sent the cast of the interior, an end view showing its convexity, also a portion of the cast of the 
outside. The impressions of numerous teeth along the hinge were very perfect, and they 
resemble those of the genus Area, to which I suppose the fossil belongs. 
Impressions of very large shells of the genus Pecten were very numerous. Species identical 
with, or similar to, that represented by fig. 77, Plate VIII, (Pecten Nevadanus) were the most 
common. The shells were but slightly convex, but appeared to have been very thin, and to 
have had very prominent ribs. The fine cast of Pecten catillifornis, represented on Plate IX, 
fig. 83, was found on a slab of sandstone, but without the impression of the exterior. It is 
more convex than the other species of Pecten, and has much larger ribs, as shown by their 
imprints on the margin. 
Two small teeth of sharks were found imbedded with the casts of these shells ; but a much 
greater number were obtained from the surface, on the tops of the hills, four or five hundred 
feet higher up in the series of beds. They were lying loosely on the ground, and appeared to 
have been washed out of the upper strata of light clay by the rains. They occur of various 
sizes and forms, some of which are shown in outline in the figures, and in full on Plate I. 
TEETH OF SHARKS—FOSSIL. 
1, 2. Oxyrhina plana, Agass. 4, 5. Galeocerdo productus, Agass. 
3. Hemipristis heteropleurus, Agass. 6. Oxyrhina tumiila, Agass. 
These fossil teeth were submitted to Professor Agassiz, and his descriptions of them are pre¬ 
sented in full in the Appendix. He observes that most of them belong to the “family of 
