Note B — PHYSICAL HISTORY OF THE VALLEY OF THE AMAZONS.

I am aware that Bates mentions having heard, that at Obydos calcareous layers, thickly studded with marine shells, had been found interstratified with the clay, but he did not himself examine the strata. The Obydos shells are not marine, but are fresh-water Unios, greatly resembling Aviculas, Solens, and Arcas. Such would-be marine fossils have been brought to me from the shore opposite to Obydos, near Santarem, and I have readily recognised them for what they truly are, fresh-water shells of the family of Naiades. I have myself collected specimens of these shells in the clay beds along the banks of the Solimoens, near Teffe, and might have mistaken them for fossils of that formation had I not known how Naiades burrow in the mud. Their resemblance to the marine genera mentioned above is very remarkable, and the mistake as to their true zoological character is as natural as that by which earlier ichthyologists, and even travellers of very recent date, have confounded some fresh-water fishes from the Upper Amazons of the genus Pterophyllum (Heckel) with the marine genus Platax.