38 MUSEUM OF COMPARATIVE ZOOLOGY. [Feb. 
collection, the utmost care is taken to select those specimens 
alone, in respect to which there is no doubt whatever as to 
geological horizon and geographical position. 
In the third place, specimens are in process of selection pistes 
all the different specific groups, a single one from each locality, 
intended to make up a grand chronologic collection. The 
design of this last assemblage of fossils, is the visible represen- 
tation of what I have been wont to term life-periods. In other 
words, it is the actual exhibition of the vertical extension or 
geologic duration of each species in every locality, of which the 
Museum has, or may come to have, specimens; and so of the: 
succession in time, both of species and of all the different divis-. 
ions of the animal kingdom. ‘The indication of succession, 
with other closely allied points, being the prominent purpose of. 
this collection, ordinary specimens are used; while sometimes 
in the lack of them, or, if the species be rare and. the fossils of 
great value, simple tablets, appropriately labelled, with refer- 
ences both to the systematic and to the fanual collection, will 
be for a while employed. This, however, will only be the case 
until the deficiency can be supplied by the reception of a larger. 
amount of material; for it is desirable that each collection be as. 
full of instruction as possible, without detraction from its peer 
dominant aim. 
As this work has gone’on, two results have come to light pri 
of which is deserving of notice. I refer to the completeness and 
deficiency of the Tertiary collections belonging to the institution.. 
In my study of these organic remains, the richness of the 
Museum stores has become more and more evident, surpassing 
in some instances even the highest expectations entertained in 
regard to them. As an instance of this amplitude and fulness, 
I may cite the consolidated collection of fossil Pleurotoma,—a 
group, the specimens of which have been for the most part 
brought together and pretty thoroughly worked up, though 
many details yet remain unexecuted. In this group alone, the 
Tertiary species belonging to the Museum, and in most instances 
represented by a fair number of individuals, probably exceed 
450; while the whole number cited by S. P. Woodward, of the 
British Museum, is 800. The Cerithium group, the systematic 
arrangement of which is considerably, though by no means so 
far advanced, seems to be equally large. Many other groups, 
