ARTICLE II. 
DESCRIPTIONS OF THE FOSSIL SHELLS. 1 
BY T. A. CONRAD. 
Philadelphia, Pa., Jamiary, 1855. 
Sir : I have examined the very interesting organic remains which you collected in California, 
and the drawings of such species as were too fragile to preserve, and I herein submit a few 
remarks upon their geological relations. There appear to he several distinct groups; but I 
cannot pretend, from such scanty materials, to designate what particular formation every group 
represents. There is no obscurity resting on the deposits of Santa Barbara and San Pedro, 
which represent a recent formation, in which you inform me the remains of the mammoth occur* 
The shells are generally those which live in the adjacent waters, and indicate little, if any, 
change of temperature since their deposition. The littoral character of this formation is very 
evident. Water-worn shells and fragments show the action of the surf, whilst entire specimens 
of bivalves, and Pholadidae, and Saxicavte, remaining undisturbed in their self-excavated domi¬ 
cils, exhibit the same disposition of marine shells that is familiar to the observer on all sandy 
and argillaceous shores. They burrow in clay, mud, or sand, beyond the ordinary action of the 
surf; whilst some are scooped out by the tempest-driven surge, and others preyed upon by fishes 
and marine animals of various kinds, and are thus broken up and deposited among the living 
species. 
Of the Eocene, and the recent formation alluded to, I can speak with confidence; but the 
intermediate beds are of uncertain age. The Ostrea vespertina, Anomia subcostata, and Pecten 
deserti, occurring in the bank of Carrizo creek, are unlike any recent forms that I am acquainted 
with from the Pacific coast, but analagous to Miocene species of Virginia. This formation may, 
therefore, be regarded as of Miocene origin—an opinion in which I am confirmed by some fossils 
collected in California by Dr. Heermann, consisting of decidedly Miocene forms ; a Mercenaria , 
(M. perlaminosa,) Con., scarcely differing from a species of Cumberland county, N. J., ( M . 
Ducatelii, Con.,) a Oemoria , Pandora, and Cardita of extinct species, closely analogous to 
Miocene forms. I am inclined, also, to refer to this period a very different group from Ocoya 
creek, the forms of which you sketched in California, as the specimens were too friable to be 
preserved. I do not recognise any recent species among them, nor any contained in an Eocene 
deposit. 
The rock at San Diego is replete with shells, generally of a small size, and appears to have a 
certain paleontological relation to those of Monterey, Carmello, and those in boulder specimens 
of Oregon, collected by Townsend and Dana, which I have referred to the Miocene period. Two 
species of San Diego, if not identical, approach Oregon shells; Nucula decisa is similar to N. 
1 These descriptions were published in 1855. See the Appendix to the Preliminary Geological Report, 8vo; Washington, 
1855. 
