Fluviatile Mollusca of Trinidad. 55 
found a single derelict example near the shore, which might 
have come on drift-wood or otherwise from some other island. 
Valvata agglutinans. In my former communication in the 
‘Annals’ I described a shell under this name. It seems, how- 
ever, to be similar to the Thelidomus brasiliensis of Swainson. 
It is not a mollusk, but the larva-case of a species of Phryganea. 
Melampus coffea exists in abundance on the northern and 
eastern coasts, where MW. puszl/us and Pedipes afra will probably 
also be found. But as these shells never occur beyond the in- 
fluence of salt water, and as their geographical distribution is 
similar to that of the marine Mollusca, I have not ineluded them 
in this list. 
Of Neritina we have two marine species, viz. NV. viridis and 
N. meleagris. The latter will live where there is a considerable 
admixture of fresh water*. 
Distribution. 
The island of Trinidad is divided into two divisions, northern 
and southern. The former consists in great part of ancient 
formations, of uncertain date, chiefly of mica-schist, compact 
and crystalline limestones, and a few associated shales. These 
rocks form a high range of hills, some of which attain eleva- 
tions of 2500 to 3000 feet. A wide tract of stratified detritus, 
through which flows the river Caroni, runs from west to east for 
the greater part of the distance across the island, separating the 
northern district from the southern. This separation is con- 
tinued to the Atlantic, on the eastern side, by barren sandy 
and siliceous strata, upon which, as upon the stratified detritus 
before mentioned, no terrestrial mollusk lives. It is rather cu- 
rious that this division has been sufficient to cause a noticeable 
difference in the molluscan fauna of each district. In the table 
I have drawn up to exhibit the distribution of the species, I 
have inserted columns to show how far this is the case. From 
this table it will be seen that the molluscan fauna of the northern 
division has a greater number of species identical with or allied to 
those of Venezuela and the Antilles than that of the southern ; 
while that of the latter has a greater affinity to the fauna of the 
Guianas. This fact may not go for much, it is true, and it is 
possible that some of the species may ultimately be found to 
have a wider range; but I thought the pomt worth noting. 
Where I have deemed the species peculiar, I have taken an 
allied form for the purpose of carrying out the comparison. 
* Some living land-shells from the Antilles (Bulimus ewxilis, B. virgu- 
latus, a species of Cistula, and Macroceramus signatus) escaped from my 
vivarium on one occasion. Should these be hereafter found in Trinidad, 
they should not be confounded with the aboriginal Mollusca. 
