of the Chalk Formation. 385 
gonie, of about 24 inches in size, as well in France as in England 
and America. This is also the case with the Mexican Trigonia, 
described (Bulletin de Bruxelles, tom. vii. no. 10) by MM. Nyst 
and Galeotti, which they have named 7’. plicato-costata. It is 
not distinct from the 7rigonia crenulata of Lamarck and Agassiz, 
and when of the same size has the same number of ribs on the 
side. When thus limited and again referred to the oldest name 
proposed by Sowerby of Zrigonia aliformis, this remarkable Tri- 
gonia is dispersed over the whole extent of America, and every- 
where characterizes the middle chalk. Even the Trigonia tho- 
racica described by Morton from Alabama is now recognized by 
him as Trigonia aliformis. M. Galeotti relates that the Trigonia 
discovered and published by him occurs in the marls of a very 
extensive limestone formation in the middle of the great and 
principal Cordillera of Anahuac, twelve French miles W.N.W, 
from the town of Tehuacan in the district of Puebla, and four to 
seven thousand feet above the sea level. In that place it is so 
abundant and large that it may be considered the chief and most 
characteristic fossil of the whole formation. It is wonderful, says 
Galeotti, to find in one place such vast accumulations of fossil 
shells, fragments of so many Ammonites several feet m diameter, 
or of gigantic stems of corals ; so much so, that perhaps there is 
no other locality on the whole surface of the earth, where over 
several square miles such an immense mass of organic remains 
are dispersed. Some Ammonites are figured by Galeotti and de- 
scribed by Nyst, but neither the description nor the figure gives 
a clear view of the whole; they remind us chiefly of the Ammo- 
nites Carderoni of D’Orbigny. This same Trigonia again appears 
in South America, in the mountains of S. Fé de Bogota, whence 
they were first brought to us by Von Humboldt (Pétréf. recueillies 
en Amérique, p. 8. f.10). It occurs here dispersed over a wide 
space, from Socorro on the north to Tocaymo on the south of Santa 
Fé. It also shows the same depression of the lower border, the 
same acute angle, scarcely exceeding 60°, of the anterior and pos- 
terior margin, the same number of perpendicular plications on 
the sides, and the same crenulations on the plications. It seems 
therefore without sufficient foundation that some naturalists be- 
lieve this Trigonia to be a new species. D’Orbigny has quoted it 
with some doubt as Trigonia subcrenulata*, and Lea under the 
name of Trigonia Tocaymaanat. Since now the mountains of 
Santa Fé are proved most distinctly by the organic remains in- 
closed in the strata to belong to the middle eretaceous series, as 
I have endeavoured to prove in the description of Humboldt and 
* Coquilles de Colombie par M. Boussingault : Paris, 1842, p. 52. pl. 4. 
f. 7-9. 
+ Trans. Amer. Phil. Soc. 2nd ser. vol. vii. p. 6. pl. 9. f.8. 
Ann. & Mag. N. Hist. Ser. 2. Vol.v. 2 
