166 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



largest drainage basin of the Isthmus proper. Proceeding westward, 

 the central portion of the Isthmus is drained by the Bayano and its 

 tributaries, which, although smaller than the Tuyra drainage, is very 

 extensive. Like the Tuyra its ramifications extend nearly to the 

 Caribbean seaboard, carrying the waters into the Bay of Panama on 

 the Pacific. 



Continuing westward, the drainage of the Chagres next sets in. 

 This is like the Tuyra and Bayano, so far as it consists of numerous 

 radiating tributaries collecting into a single arterial outlet to the sea, but 

 its waters are carried into the Caribbean, while its headwaters nearly 

 extend to the Pacific. West of the Chagres to the Costa Eican boun- 

 dary the drainage consists of simpler or less complicated streams, rising 

 nearer the central or axial line and flowing into either ocean. Thus 

 it will be seen that the drainage of the larger or eastern half of the 

 Isthmus is complex, and finds its way to the ocean by concentrating 

 into three principal arterial channels, while that to the westward is 

 simple. These eastern streams are all tidal for a great distance inland 

 so that the real distance between the waters of the two oceans is much 

 less than that from shore to shore. For instance, while the actual 

 distance from the Caribbean to the Pacific along the line from Colon 

 to Panama is -47 miles, owing to the backing of the tide waters up the 

 Chagres and the Eio Grande, the actual distance of the true marine 

 base level is only a little over half that amount. 



The region as a whole, especially the Caribbean side, is covered by a 

 dense jungle of vegetation consisting of grasses, sedges, wild plantain, 

 and trees characteristic of the lower lands of the whole Caribbean coast, 

 lying below 1,000 feet. The peculiar and beautiful arborescent flora 

 which characterizes the higher slopes of Central America and the 

 northern portion of South America does not appear on the Isthmian 

 region. 



The Isthmian region differs from the Andean region to the east, not 

 only by the absence of the tremendous heights of the latter, but by an 

 entirely distinct geological structure and composition. The Andes are 

 composed of sedimentaries of the Cretaceous or earlier periods, distorted 

 into gigantic folds and intruded through by igneous rocks. There are 

 no resemblances in age, composition, or structure to this Andean type of 

 topography in the Isthmian region. 



Less easily defined, but as strikingly different, is the distinction be- 

 tween the Isthmian region and the high Costa Rican (Central American) 

 volcanic plateau lying to the west of it. These differences are not en- 





