PREFACE. 
xvii 
must select certain normal groups which do not 
approximate too closely to each other, and enumerate 
by name the species common to more than one of 
these. Thus, for example, we might omit in our 
tables the Newer Pliocene formations altogether, and 
enumerate the shells common to the Recent and Older 
Pliocene beds. 
I have arranged the tertiary formations in four 
groups, as I had determined to do before I was 
acquainted with M. Deshayes ; and in his tables he 
has referred the shells to three periods, according to 
which he had classed them before he had any commu- 
nication with me. No confusion, however, will arise 
from this want of conformity between the tables and 
my classification, since I have named two of my 
periods (the Newer and Older Pliocene) as subdivisions 
of one of his ; and by reference to the Synoptical 
Table, at p. 61, the reader will see which localities 
mentioned in M. Deshayes's Tables belong to the 
Newer and which to the Older Pliocene period. 
In the summer of 1831 I made a geological 
excursion to the volcanic district of the Eifel, and on 
my return I determined to extend my work to three 
volumes, the second of which appeared in January, 
1832. The last volume has been delayed till now by 
many interruptions, among which I may mention a 
tour, in the summer of 1832, up the valley of the 
Rhine, when I examined the loess (vol. iii. p. 151), 
Vol. III. c 
