144 M. de Buch on the 
the great transition nucleus of Titicaca, which penetrates 
with the ancient rocks into the chain of the Andes. Among 
these forms, the most remarkable is the beautiful univalve 
which M. de Humboldt first brought from San Felipe, to the 
south of Quito, near the Amazon river, and to which I have 
given the name of Pleurotomaria Humboldtii ; M. d’Orbigny, 
and Darwin after him, calls it Turritella Andii, although it 
is still doubtful whether this is done with propriety. It seems 
to be quite peculiar to South America. Darwin likewise met 
with it in abundance in the formations which extend from 
Coquimbo to Rio Claro and Arqueros, and even beyond Guasco 
and the Amolanas, the principal valley of Copiapo. This 
Pleurotomaria is always associated with the pectens, which 
are likewise found in the northern portion, between the Mon- 
tar and Guancavelica, in such great abundance as to form 
fields of fossils and entire mountains, which have been long 
and generally known under the name of Choropampas (Pecten 
alatus Dufresnoyi, d’Orb.). It is the same which Ulloa in 
1761, places, to his great astonishment, at so great a height 
above the level of the sea, where it seems to compose moun- 
tains of shells. This astonishment was expressed in all 
books till it was perceived that it was not absolutely neces- 
sary that the shells should have lived at these heights, but 
that they may have been raised upwards from the bottom of 
the sea. As the Hyppurites organisans (d’Orbig., p. 107, 
t. 22) is met with in the middle of the beds of Pectens, it 
follows that all these beds in Perou as well as at Coquimbo 
and Copiapo, belong at least to the gault, an opinion which 
appears clearly justified by an Exogyra which M. Domeyko has 
sent to Paris. It is, in fact, in every respect like that which 
has been described and figured by Morton, Gryphea (£xo- 
gyra) Pitscheri of the Texas, the position of which M. Ferdi- 
nand Romer has placed above the gault, at Friedrichsberg. 
The deeper chalk strata, analogous to those of Aconcagua, 
are not, however, unknown in the Andes of Lima. The cele- 
brated zoologist, M. de Tschudi, has found on the eastern ac- 
clivity of the mountains, between Oroja and Yaui, near 
Tarma, and after him many other travellers have likewise 
found neocomian shells well characterised. Pterocera Eme- 
