258 Guppy— Tertiaries of Trinidad. 
single tooth has come to light. There are also remains of Cirripedes 
(Balani), and fragments of Crustacea (brachyurous decapods) ; also 
a few spines and fragments of Echinoderms. 
The Shells marked* in the preceding list are known to me, by 
recent examples, to be still existing in neighbouring seas. Those 
marked} are species otherwise unknown to me, either by books or 
specimens. Ostrea cucullata is, I believe, a shell of the eastern 
seas. Cardium (Papyridea), sp., is allied to the recent C. (Papy- 
ridea) ringiculum, but is much smaller and thicker. 
It will be observed that there are 27 species known as existing 
in contiguous waters, out of a total number of 56 Gasteropods, 
including two freshwater shells and two opisthobranchs. Then 16 
of the remainder, if not found in the surrounding seas, are probably 
existing elsewhere. To two of these I have assigned the names 
WNassa incrassata and Trochus granulatus, on account of their 
resemblance to the European species of those names, but at the same 
time with much hesitation. 
The proportion of recent species thus arrived at is nearly 80 per 
cent.; or 20 per cent. of unknown and extinct species. Making 
due allowance, however, for imperfect knowledge, there would pro- 
bably remain at least 10 per cent. of extinct species of Gasteropoda. 
With regard to the Conchifera, the proportions are nearly the 
same. ‘There are 22 species, out of the total of 36, which are cer- 
tainly known to me to exist in contiguous seas. To this number 
may be added four species, some of which probably exist elsewhere. 
Nucula similis and Corbula pisum are species of the European 
HKocene. I can detect no difference between the examples from 
Matura and the English species; but some doubt, of course, rests on 
the determination, owing to the distance both of locality and of time. 
I think, from what has been stated, that we may consider it mode- 
rately safe to infer, that of the fossil Mollusca of the Matura deposit 
there is a percentage of at least 10 extinct species. This would 
bring the deposit within the Pliocene Period according to the classi- 
fication of Lyell;* and, in searching for European equivalents, we 
should probably find that the Glacial deposits of Europe present the 
closest analogies with the Matura beds. 
I may remark, by the way, that the fact has not practically been 
overlooked by Geologists, that even where all the species are recent, 
yet, if some of them are only found in distant seas, and exist under 
different climatal conditions to what those occurring in the localities 
where such species are found fossil lived under, the differences between 
the recent and fossil faune mark a real progress in geological time, 
and entitle the strata from which the fossils are obtained to a dis- 
tinctive name. ‘The Matura deposits are, however, less remarkable 
in this respect than in regard to the small size of the Shells found 
in them; a point which will be dwelt upon in the following sections 
of this paper. 
_§ 3. Conditions of Deposit.—It is probable, from the sandy nature 
* Lyell, Manual of Elementary Geology, 5th ed., p. 105, and Supplement, p. 13. 
