26 PROCEEDINGS OF THE ACADEMY OF 



species of that genus. So partial a view of the hinge has been 

 obtained that it cannot be certainly determined as indicating that 

 genus. The shell described as Tellina b^- Mr. Gabb is a young 

 valve of Pachydon teimis. So that any certain evidence of a 

 marine origin for the Pebas group is wanting. 



Professor "Woodward remarks that in the living analosrue of 

 Pachydon, " Azara or Potamomya, we have just the evidence we 

 need to argue upon" in respect to the nature of the habitat of the 

 Pebas shells, and quotes Darwin's observations on Azara lahiata. 

 Darwin in his " Geolog-ical Observations on South America" saj's, 

 " On the northern bank of the great estuary of the Rio Plata, 

 near Moldonado, I found at the head of a lake, sometimes brack- 

 ish, but generally containing fresh water, a bed of muddy clay 

 containing Azara labiata,^' etc. This representation of the habits 

 of that shell indicates, I think, those of the Pebas fossils, which 

 may have inhabited a lake similar to that of Moldonado. 



The following letter from Mr. Steere to Prof. Hilgard explains 

 the position of the fossils : 



" I first saw Prof. Orton's fossil bed near Tabatinga. It is com- 

 posed of horizontal beds of blue clay, separated b}'^ beds of dirty 

 coal, that seems more like peat than coal. There seems to be 

 nothing regular in these beds, thus differing apparently in thick- 

 ness, and appeared in a distance of a few hundred yards. I have 

 sent you fossils from these different localities, Pebas, Old Pebas, 

 and Pichana. The}' are situated in relation to each other some- 

 what as follows : Pebas one mile from the mouth of the Ambyacu 

 River, and Old Pebas two miles below, and Pichana perhaps 

 fifteen miles from the mouth of the Ambyacu. The fossils from 

 Pebas are 60 or 10 feet above the river level, while at Old Pebas 

 thej' ai'e at the water level, though tlie shell beds at Pebas ma}' 

 extend down, as the lower strata are covered by fallen and washed 

 earth, leaving the fossil beds visible only at the top. The fossil beds 

 appear to have been denuded irregularly and then covered with a 

 bed of red and white clay and sand, much like that formed so 

 plentifully on the Lower Amazon in some places. This shows in 

 the river bank 30 feet thick, but at Pebas only 8 or 10. I made 

 a rough section of the exposed beds at Pebas, with the following 

 i-esult though the result I think is of little importance, the beds 

 change so frequently. 



