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For instance, some species of Trigonia, a great genus of shells in the secondary formations, survive in the Australian seas; and a few members of the great and almost extinct group of Ganoid fishes still inhabit our fresh waters. Therefore, the utter extinction of a group is generally, as we have seen, a slower process than its production.
Occasionally, when the force of the river abated, marine shells of the genera Trigonia, Turritella, Pecten, etc., established themselves in the same area, and plants allied to Zostera and Fucus grew on the bottom. The fossil plants of this member of the upper chalk at Aix have been diligently collected and studied by Dr.
Arca, perhaps A. Gabrielis, d'Orbigny, "Pal. Franc." Mr. Pentland made a collection of shells from this same spot, and Von Buch considers them as consisting of: Trigonia, resembling in form T. costata. Pholadomya, like one found by M. Dufresnoy near Alencon. Isocardi excentrica, Voltz., identical with that from the Jura.
I must, however, again call attention to the fact that the Cardium auca is found both at Concepcion and in the undoubtedly tertiary strata of Coquimbo: nor should the possibility be overlooked, that as Trigonia, though known in the northern hemisphere only as a Secondary genus, has living representatives in the Australian seas, so a Baculite, Ammonite, and Trigonia may have survived in this remote part of the southern ocean to a somewhat later period than to the north of the equator.
Some of the fragments of Ammonites were as thick as a man's arm: the Gryphaea is much the most abundant shell. These fossils M. d'Orbigny considers as belonging to the Neocomian stage of the Cretaceous system. Dr. Trigonia costata, identical with those found in the upper Jurassic beds at Hildesheim. 3. Pecten striatus, identical with those found in the upper Jurassic beds at Hildesheim. 4.
Mactra Araucana, d'Orbigny, "Voyage, Part Pal." 12. Arca Araucana, d'Orbigny, "Voyage, Part Pal." 13. Nucula Largillierti, d'Orbigny, "Voyage, Part Pal." 14. Trigonia Hanetiana, d'Orbigny, "Voyage, Part Pal." During a second visit of the "Beagle" to Concepcion, Mr.
In illustration of shells having a great vertical range, it may be stated that in England some few species pass up from the Lower to the Upper Oolite, as, for example, Rhynchonella obsoleta, Lithodomus inclusus, Pholadomya ovalis, and Trigonia costata. Ammonites macrocephalus, Schloth. One-third natural size.
In the South Australian province species of Mactra, Psammobia, Venus, Tapes and Pecten, all peculiar, were taken. This is the region of the peculiar genus Myadora, of which five species were dredged on sand in 6 fathoms at Port Jackson, along with Myochama anomioides, Trigonia margaritacea, Lima bullata, and Cardium radiatum.
Two of these shells, namely, the Gryphaea and Trigonia, appear to be identical with species collected by Meyen and myself on the Peuquenes range; and in the opinion of Von Buch and M. d'Orbigny, the two formations belong to the same age.
Kent collected for me some silicified wood and shells out of the concretions in the sandstone from Tome, situated a short distance north of Lirguen. They consist of: Natica australis, d'Orbigny, "Voyage, Part Pal." 2. Mactra Araucana, d'Orbigny, "Voyage, Part Pal." 3. Trigonia Hanetiana, d'Orbigny, "Voyage, Part Pal." 4.
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