Structure of the Amazons Valley. 



367 



sand ; next, several feet of pebbles ; and then blue clay con- 

 taining shells. From the collections made at these localities, 

 the following thirty species have been determined : — 

 Bivalves. Univalves. 



Pachyodon carinatus, Conrad. 



obliquus, Gabb. 



tenuis, Gabb. 



erectus, Conrad. 



cuneatus, Conrad. 



ovatus, Conrad. 



cuneiforrnis, Conrad. 



dispar, Conrad. 



Dreissena scripta, Conrad. 

 Anodon Batesii, Woodward. 



pebasana, Conrad. 



Triquetra longula, Conrad. 

 Ostomya papyria, Conrad. 

 IIaplotha3rus capax, Conrad. 



Isaea Ortoni, Gabb. 



lintea, Conrad. 



Liris laqueata, Conrad. 

 Ebora crassilabra, Conrad. 

 Nesis bella, Conrad. 

 Neritina Ortoni, Conrad. 

 Dyris gracilis, Conrad. 

 Heniisinus sulcatus, Conrad. 



Steerei, Conrad. 



Iquitosa tuberculifera, Conrad*. 

 Pachytoma tertiana, Conrad. 

 Toxosoma eborea, Conrad. 

 Cirrobasis venusta, Conrad. 

 Liosoma cnrta, Conrad. 

 Cyclocbeila pebasana, Conrad. 

 Bulinms linteus, Conrad. 



These interesting fossils have attracted much attention by 

 their extraordinary character, and by the light which they throw 

 upon the largest continuous Tertiary formation in the world. 

 All the species and twelve of the genera are extinct. The 

 impalpable clay in which they were imbedded was admirably 

 fitted for their preservation. Some have retained their colours 

 and epidermis ; and the bivalves generally occur with the valves 

 united and closed. They exist also in such vast numbers that 

 they must have lived and died on the spot. The bivalves 

 are most abundant at Pichana, and the univalves at Iquitos — 

 localities at least 150 miles apart : the former may be the lower 

 stratum, and the other the upper. The Hemisinus is particu- 

 larly abundant at Iquitos, and very rare in the Pebas district. 

 Mr. Gabb led me astray in saying that these shells are marine. 

 Most of them are freshwater ; many are estuarine (but might 

 have lived in fresh or brackish water) ; and a few are terres- 

 trial. Mr. Conrad, who examined my large collections, and 

 is better prepared to speak than any other palaeontologist, 

 considers the beds Eocene^. 



* This beautiful and characteristic shell was originally described, in 

 Proceed. Acad. Nat. Sci. Philad. vol. xxvi., as a Hemisinus; but Mr. 

 Conrad has since decided that it belongs to a new genus, distinguished 

 by its high Melania-like spire and short patulous aperture. " Subulate, 

 subtuiTeted ; whorls numerous, spirally ribbed ; aperture short, oval ; colu- 

 mella regularly arched, solid, subtruncated at base ; outer lip regularly 

 curved." The name is derived from Iquitos, Peru, where it is very 

 abundant. Hemisinus and Triquetra are characteristic genera of South- 

 American rivers. 



t Per contra, Professor Hartt, who has never seen the Maranon, de- 

 cides " that it was in the latter part of the stage of growth of the basin 

 that the clays of the Upper Amazon were deposited and the Pebas 

 shells lived. This appears to have been near the close of the Tertiary." 



