NATURAL SCIENCES OF PEIILADELPHIA. 25 



REMARKS ON THE TERTIARY CLAY OF THE UPPER AMAZON, WITH 

 DESCRIPTIONS OF NEW SHELLS. 



BY T. A. CONRAD. 



Professor Eug. W. Hilgard has sent me for examination a 

 number of fossils collected on the Upper Amazon by Mr. Steere. 

 The extraordinary character of this interesting group has made it 

 one of more than ordinary attraction. The species and even 

 many of the genera being limited to these deposits, we fail to find 

 a point of comparison with other groups of fossils. Some natural- 

 ists have supposed from the look of freshness and the perfection 

 of many specimens, that they may be of comparatively^ modern 

 origin, but the clay in which they are imbedded is admirably 

 fitted for their indefinite preservation. The clay is generally free 

 from iron, and thus one source of injury is wanting. Tlie colors 

 of the shells are sometimes preserved in perfection, and even the 

 epidermis of a few specimens of Pachydon, and also portions of 

 cartilage, but I do not regard these as evidence of very late de- 

 position of the group. The colored markings of a cretaceous 

 Neritina from Mississippi, and on a univalve of the eocene of 

 Alabama, as well as portions of cartilage in an eocene bivalve, all 

 of which I have seen, might as well be considered evidence of late 

 tertiary deposition of the beds in which they occur. Were the 

 Pebas group of pliocene date, we should expect it to contain 

 man}' recent species of the Lower Amazon, and especially Azara, 

 but as far as we yet know, such shells are absent. Triquetra and 

 Hemisinus are characteristic genera of South American rivers, 

 but the fossil species are not identical with living ones. Mr. Dall 

 informs us that " many of the genera are exclusively marine," but 

 I am sure no such genus was in the collection which he studied at 

 the time. Only one doubtful genus of this character has been 

 found in all the collections since made. The fresh-water shells 

 consist of, 2 Anodontas, 2 of Hemisinus, and 2 of Triquetra, 

 besides several land shells, while the estuary genei*a, Mytilopsis 

 and Neretina, might have lived in either fresh or brackish water. 

 On the coasts of the Atlantic States of North America these two 

 genera live in the latter. The only shell which might be sup- 

 posed to be strictly marine is a Nuculana, if it prove to be a 

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