2 PROCEEDINGS OF THE NATIONAL MUSEUM vol.82 



tributary of the Rio Paute, is the nearest large town, about 4 or 5 

 miles to the west. The Eio Matadero rises in the Cajas lakes, on the 

 divide, probably the extreme western reach of the Atlantic drainage 

 S3^stem in southern Ecuador, and perhaps in all South America. 

 These are the nearest lakes of the Atlantic system, and consist of a 

 number of large bodies of w^ater in lofty valleys in the passes of 

 Cajas. To the southwest are many small rivers, tributaries of the 

 larger coastal rivers of the Pacific drainage.^ 



At present there are no reliable data to establish the age of the 

 deposits, but as none of the species and none of the genera except 

 Pomacea occur in the recent fauna, it is probable that the age can 

 not be later than Pliocene, and it may be earlier. 



The formation is exclusively of fresh-w^ater origin, as all the species 

 found require a fresh-water habitat. In a former paper the senior 

 author (Marshall, 1928) described and discussed several new genera 

 and species of pearly fresh-water mussels from Pebas, Peru. Other 

 authors had already described from that formation various land, 

 fresh-water, and brackish-water shells, and a few which are doubt- 

 fully classed as marine. The land shells must, of course, have been 

 washed in from the countryside. 



Pebas is about 325 feet above sea level. Biblian is at least 8,000 

 feet above the sea and by air-line is only about 450 nautical miles 

 from Pebas, although by the tortuous rivers it may be twice that far. 

 The occurrence of fresh- water, brackish-water, and marine (?) species 

 at Pebas suggests the possibility that formerly the locality was occu- 

 pied by an estuary in which the brackish-water and marine species 

 found a natural habitat. The inner reaches of the estuary may have 

 been suitable for fresh-water species, just as is the case with the 

 Rio de la Plata, in which very fine specimens of fresh-water mussels 

 are found living, especially along the shores of Colonia. It is pos- 

 sible also that the species were washed down from fresh waters of 

 higher levels. Investigation of the region between Pebas and Biblian 

 may yield other fresh-water deposits, which will explain the mixed 

 fossil molluscan fauna found at Pebas. As the fossil fresh-water 

 species found at Biblian and Pebas seem to be of the same geologic 

 age and as the two localities lie on the same stream, it appears likely 

 that there is an intimate geological relationship existing between the 

 two places. The presence of Pomacea in the deposit at Biblian may 

 indicate a quiet-water habitat, as the recent species of the genus de- 

 light in placid waters. 



1 Most of the above notes were obtained from charts and relief models owned by the 

 Pan American Union. An instructive map of the watershed of Ecuador was published 

 recently by Edward W. Berry (1929, p. 80). 



