The upper beds in this section probably correspond with parts of the great gypseous formation; and the lower beds of red sandstone conglomerate and fossiliferous limestone no doubt are the equivalents of the Hippurite stratum, seen in descending from Arqueros to Pluclaro, which there lies conformably upon the porphyritic conglomerate formation. The fossils found in the third bed, consist of:—
Pecten Dufreynoyi, d’Orbigny, “Voyage, Part Pal.” This species, which occurs here in vast numbers, according to M. D’Orbigny, resembles certain cretaceous forms.
Ostrea hemispherica, d’Orbigny, “Voyage” etc.
Also resembles, according to the same author, cretaceous forms.
Terebratula aenigma, d’Orbigny, “Voyage” etc. (Pl. 22 Figures 10-12.)
Is allied, according to M. d’Orbigny, to T. concinna from the Forest Marble. A series of this species, collected in several localities hereafter to be referred to, has been laid before Professor Forbes; and he informs me that many of the specimens are almost undistinguishable from our oolitic T. tetraedra, and that the varieties amongst them are such as are found in that variable species. Generally speaking, the American specimens of T. aenigma may be distinguished from the British T. tetraedra, by the surface having the ribs sharp and well-defined to the beak, whilst in the British species they become obsolete and smoothed down; but this difference is not constant. Professor Forbes adds, that, possibly, internal characters may exist, which would distinguish the American species from its European allies.
Spirifer linguiferoides, E. Forbes.
Professor Forbes states that this species is very near to S. linguifera of Phillips (a carboniferous limestone fossil), but probably distinct. M. d’Orbigny considers it as perhaps indicating the Jurassic period.
Ammonites, imperfect impression of.
M. Domeyko has sent to France a collection of fossils, which, I presume, from the description given, must have come from the neighbourhood of Arqueros; they consist of:—