350 THE AMERICAN NATURALIST. [Vou. XXXIII. 
The same number contains an elaborate paper on the life history of 
Lemna minor, by Otis W. Caldwell. 
Möllers Deutsche Gartner-Zeitung of January 14 contains an articl 
on Mexican orchids in their native home, by Othon Krieger, which | 
is illustrated by reproductions of two photographs presenting a very | l 
vivid picture of the abundance of these epiphytes of our plant-houses 
and of the difficulties attending their collection. 
Botanists who may make the acquaintance of the white ash of 
Australia will not find a species of Fraxinus, but a Eucalyptus, which 
has been described and figured by Deane and Maiden in No. 91 of 
the Proceedings of the Linnean Society of New South Wales under the 
name Æ. fraxinoides. The same number contains descriptions and 
figures of two additional species of Eucalyptus, by R. T. Baker. 
GEOLOGY. 
The Isthmus of Panama, as has been shown by a geological recon- 
noissance made by R. T. Hill,’ is an ancient mountain range much 
reduced in height and deeply dissected by erosion. The drainage 
system is well developed and consists of several principal streams, 
which have many rapidly flowing branches near their sources, but 
comparatively long, low-grade trunks into which the tides extend 
many miles from the sea. The larger streams generally are charac- 
terized by drowned mouths and actively corrading head waters. 
There is an absence on each border of the isthmus of a coastal 
plain, similar to that on the Gulf and Atlantic borders of the United 
States. The uplands come boldly down to the sea in a series of 
bluffs and headlands, separated by the partially drowned valleys. 
On each shore, however, there is a submerged platform, which 
extends out to about the 100-fathom line, where the bottom descends 
rapidly into water of great depth. The topography of these sub- 
merged shelves indicates that they were formerly coastal plains across 
which the present streams were extended and excavated channels. 
A downward movement of the land has submerged the former coastal 
plain and given origin to coastal swamps, and permitted the encroach- 
ment of the sea far up the ancient, low-grade valleys. 
1 Hill, R. T. The Geological History of the Isthmus of Panama and Portions 
of Costa Rica. Based on a reconnoissance made for Alexander Agassiz. Bulletin 
of the Museum of Comparative Zoölogy at Harvard College, vol. xxviii (1898), 
-= No. 5, pp. 151-285, with 19 plates. 
