eigenmann: the FRESHWATER FISHES OF BRITISH GUIANA 95 



Between the Pakatuk Cataract and Amatuk the river is again navigable for 

 small boats. The "Amatuk Falls are over some of the lower beds of the great 

 sandstone and conglomerate formation. Here the sandstone and conglomerate is 

 fine grained, with occasional quartz-pebbles, of a red color, and shows very clearly, 

 and in many places markedly, current-bedding. Not more than about twenty-five 

 feet of the lower beds are exposed in the actual section of the falls. A sill of diabase 

 intrusive through the sandstone causes small rapids above the main fall." 



At Waratuk (a few miles above Amatuk) a dike of diabase causes rapids, and as 

 stated, another series of cataracts occurs at Tukeit and between Tukeit and the 

 foot of the Kaieteur. 



Above the Kaieteur, for about forty miles as the river flows, the Potaro is 

 navigable to bateaus without interruption. At Aruataima at the end of this stretch 

 is another cataract and beyond this point I did not go. 



As stated, one of the chief objects of the expedition was to study the relation 

 of the faunas of the upper to that of the lower Potaro. 



The questions of prime importance concerning the fauna of the upper Potaro, 

 i. e., the Guiana Plateau, are: 



1. Of what does it consist? 



2. Whence did it come? 



3. How did it get there? 



The first and second of these questions are simplified as to fishes by the obvious 

 fact that fishes live in water, and that most of them die in a few moments after 

 being taken from the water. Their migrations must be along waterways; i. e., 

 along well-defined and restricted channels. 



1. Fishes Taken in the Potaro. 

 The species found both above and below the Kaieteur are given in italics. 

 Those confined to the plateau are in heavy-faced type. These two categories 

 answer specifically the first of the above questions. Species peculiar to the Potaro 

 are marked with an asterisk, those genera peculiar to it with a double asterisk. 

 Those found between the mouth and the cataracts above Potaro Landing are 

 marked A, those between the latter place and Amatuk, B, and those above the 

 Amatuk Cataract, C. 



AspredinidjE. 3. Pseudopimelodus villosus A 



1. Bunocephalus chamaizelus A B 4.*Pseudopimelodus albomarginatus C 



Silurid^e. 5.*Brachyglanis frenata B 



Pimelodince. 6.*Brachyglanis phalacra B 



2.**Megalonema platycephalum A 7.*Myoglanis potaroensis ABC 



