Guppy— Tertiaries of Trinidad. 259 
of the beds, that they were not thrown down at any very great dis- 
tance from land; while it could hardly have been as a beach that 
the deposit was accumulated. The Shells are many of them too 
fragile to have withstood the attrition which accompanies exposure 
on abeach. We must look, therefore, for a condition intermediate 
between the two as probably the nearest to the truth ; and we find 
that the fauna is one which would, in most respects, suit a depth of 
from 20 to 50 fathoms. : 
On those tropical beaches where Univalves predominate, we have 
Litorina, Patella, and Nerita. None of these are represented in 
the Matura beds. On the other hand, on those long stretches of 
sandy beach in the tropics where the molluscan fauna is chiefly 
bivalve, we find Mactra turgida, M. carinata, Trigona mactroides, 
Donax denticulata, D. striata, and, more rarely, Venus granulata, 
Cytherea dione, and a few Tellens. In the deposits at Matura we 
have, however, Shells characteristic of a certain depth of water, 
such as Yerebra, Oliva, Marginella, Conus, Erato, Chemnitzia, 
Eulima, Odostomia, Cecum, Dentalium, &c., and several of the 
Conchifera. The only strictly littoral Gasteropod in this collection 
—namely, Chiton—is represented by a solitary plate. As to the two 
freshwater species, each of which is represented by a single example, 
they may have been borne out, on floating wood or otherwise, from 
some stream of the neighbouring land. 
The comparative numbers of the individuals of each species found 
in the fossil and recent conditions is another matter which deserves 
some consideration, though I cannot make many complete and accu- 
rate observations on that head. The little Rice-shell, Oliva oryza, 
seems to have been almost as common during the deposition of the 
Matura beds as at present; while the Astarte (Gouldia) Martini- 
censis is much more rare now than it appears to have been formerly. 
This small shell occurs in immense numbers in the Matura deposit. 
Corbula pygmea is not quite so numerously represented ; it is by no 
means rare in some localities at the present day, but it is second 
only to the Astarte before mentioned, in point of numbers, in the 
Matura deposit. 
§ 4. Climate of the Period of Deposition.—Bearing in mind what 
has been said respecting the proportion of recent and extinct species, 
it will appear that these deposits probably belong to an epoch not 
far removed from that of the Glacial and Preglacial beds of Europe. 
But the extremely small size of the fossils found at Matura is one 
of the most remarkable features of the deposit. Even where the 
Shells belong to recent types of average size, the fossil represen- 
tatives are almost invariably dwarfed. ‘There are only one or two 
exceptions to this rule; e.g., Cardium isocardium, Turritella imbri- 
cata, and Bulla striata, which attain an ordinary, but not a large, 
rowth. 
Where the fossil Shells belong to recent types of small size and 
Arctic genera, they are not smaller than their living representatives. 
As examples of this, I may cite Astarte (Gouldia) Martinicensis, the 
Corbule, the Leda, and the Nucule. 
$2 
