572 
cal Miocene contains but 17 per cent. and the Pliocene from 
35 to 50 per cent. of living species. But that rule, while i: 
applies perfectly well to the local deposits on which it was 
based, is too empirical to be followed elsewhere, except in a 
very general manner and where the other data are in accord. 
An essential objection to the numerical rule exists in the 
different values that students place on specific characters. 
No two writers agree on this subject. Besides, as regions 
become more thoroughly worked up, discoveries of additional 
fossils, or the finding of living species, previously known 
conly as fossils, vary the proportions constantly. The general 
deductions, therefore, drawn by an experienced palseontolo- 
gist from large collections, are safer guides than any table 
of percentages 
fem, 
Mus. Comp. Zoouoey, \ 
Cambridge, Mass., Dec. 3, 1872. 
My Dear Pror. LEsLey: 
The steamer did not sail on Saturday and I 
have availed myself of the delay to run up here. It was very fortunate, 
since I have had the opportunity of seeing Dr. G. A. Maack, and of 
learning from him some of his geological results on the late Selfridge 
Expedition on the Isthmus. Please have the following. note added to 
my paper, with the permission of the Society : 
The results of the explorations of Dr. Maack last year, on the Isthmus 
of Darien, put at rest the question of the late geological origin of the 
Isthmus. He found three late Tertiary strips extending entirely across, 
proving three channels at least in the Miocene, and some of the deposits 
indicate a much later era of elevation. One of these, 10 miles inland 
from Panama, evidently Post Pliocene, is at least 150 feet above the tide. 
In a very cursory examination of his fossils I detected the following 
species, also found in Santo Domingo : 
Melongena melongena. 
Murex recurvirostris. 
Malea ringens. 
Terebra robusta. 
Conus pyriformis. 
Natica sulcata. 
Jerithium plebium. 
Turritella. 
Cypreea exanthemata (v. cervinella). 
Venus paphia, 
Jardium Haytense. 
Pecten papyraceus. 
Dr. Maack in his report calls the older beds of Panama, Pliocene. They 
