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 tho 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 Rican 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 Rio 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- 
