204 
PHYSICAL HISTORY OF THE 
order to accumulate formations of such a thick¬ 
ness, should not contain numerous remains 
of the animals formerly inhabiting it.* The 
only fossil remains of any kind truly belong¬ 
ing to it, which I have found in the formation, 
are the leaves mentioned above, taken from 
the lower clays on the banks of the Solimoens 
at Tonantins ; and these show a vegetation sim¬ 
ilar in general character to that which prevails 
there to-day. Evidently, then, this basin was 
* 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 ma¬ 
rine, but are fresh-water Unios, greatly resembling Aviculas, 
Solens, and Areas. Such would-be-marine fossils have been 
brought to me from the shore opposite to Obydos, near San- 
tarem, and I have readily recognized them for what they 
truly are, fresli-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 Telfe, 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 Pterophyl- 
lum (Heckel) with the marine genus Platax. 
