192 



AMERICAN JOURNAL 



DESCRIPTIONS OF NEW FOSSIL SHELLS OF THE UPPER 



AMAZON. 



BY T. A. CONRAD. 



Mr. Gabb has described and figured in the fourth volume of 

 the American Journal of Conchology a few species of fossil shells 

 from a tributary of the Upper Amazon near Pebas. A larger 

 number of species from the same group, collected by Mr. Haux- 

 well, has been sent to me by Professor James Orton, of Vassar 

 College, to describe. 



Prof. Orton remarks, " a very few of these shells were found 

 where I first discovered the deposit, which was at Pebns, near 

 the mouth of the Ambiyacu ; but the rest, comprising nearly the 

 whole collection, were obtained nearly 30 miles below Pebas, on 

 the south side of the Marafion, at Pichua, just west of Cocha- 

 quinas. The locality is about 2200 miles up the Amazon, and 

 the shells appear to be more abundant even than at Pebas. They 

 occur in that peculiar formation of fine laminated colored clays 

 which is spread over the entire valley of the great river, and 

 which Prof. Agassiz had pronounced ' Drift.' Similar fossilife- 

 rous beds the natives say are to be seen at Omagua, and also up 

 the Ambiyacu." 



There are 7 species of the genus Pachydon, Gabb, which does 

 not, so far as we know at present, have any living representative, 

 and is very different from any known existing fresh water genus. 

 In the collection are fragments of a singular bivalve, probably 

 allied to Mulleria, one of which is pearly as a Uriio and has a nar- 

 row elongated muscular impression, very different in size and 

 outline from that of Mulleria. It seems to have been carried by 

 a flood in some river of the time to its present position. 



It is not possible to state without doubt what the relative 

 stratigraphical position of this group may be, but if all the spe- 

 cies are extinct it cannot be later than the Tertiary. The spe- 

 cies being all new, or at least unknown in the books and cabinet 

 of the Academy of Natural Sciences, the Pleistocene origin of 

 the group is at least very doubtful. It may have lived either in 

 fresh or brackish water, but it is certainly not of marine origin^ 



