INTRODUCTION. 
GENERAL TOPOGRAPHY AND HyDROGRAPHY OF PANAMA. 
In the region of the Canal Zone the Atlantic slope is very much 
broader than the Pacific slope. Following the line of the Panama 
Canal, it is about 40 miles from the Atlantic coast to the crest of the 
divide and only 10 miles from thence to the Pacific coast. This, in 
the Canal Zone region, where the rainfall is heavy,* gives rise to the 
comparatively large Rio Chagres on the Atlantic slope opposed by 
rather small coastal streams on the Pacific slope, namely, the Rio Chor- 
rera, Rio Grande, Rio Marte Arnade, Rio Abaco, Rio Juan Diaz and 
other small streams. 
Going eastward or toward Colombia the Pacific slope becomes 
broader and the Atlantic slope narrower. The increased width of the 
Pacific slope then gives rise to the rather large Rio Bayano, the mouth 
of which is about 30 miles east of Panama City, and to the still larger 
Rio Tuyra which empties into San Miguel Bay, about 100 miles south- 
east of Panama City. Opposite these streams, on the narrow Atlantic 
slope, there are only very small coastal streams. 
In western Panama the watershed more nearly follows the median 
line of the Isthmus and no large streams occur on either slope. 
There are then three river basins of considerable size in Panama, 
viz.; the Rio Chagres on the Atlantic slope and the Rio Bayano and the 
Rio Tuyra on the Pacific slope. Collections were made in all of these 
streams. The Rio Chagres with its numerous tributaries was quite 
thoroughly explored, and it is believed that our collections contain, 
with perhaps very few exceptions, all the species occurring there. Our 
explorations in the Rio Bayano and the Rio Tuyra basins were not so 
thorough, and our collections from these rivers are certainly not ex- 
haustive, but sufficient to indicate the nature of their faunas. Besides 
these large streams, the small streams opposite the Rio Chagres and 
as far westward as the Rio Chame on the Pacific slope were visited and 
collections made in each. On the Atlantic slope only a few of the small 
coastal streams, two at Porto Bello and two at Toro Point, were visited. 
This then leaves many small streams of western Panama unexplored. 
*The rainfall in the Canal Zone region is heaviest on the Atlantic slope and 
i at Porto Bello where during a single day of 24 hours as much as II 
inches of water are known to have fallen. In 1909, 58.07 inches of rain fell during the 
month of December and the total rainfall for the year was 237.28 inches. 
223 
