GENEEAL EEPOET ON INVESTIGATIONS IN POETO EICO. 
25 
burrow into rocks or live among their interstices. Bivalve shells are rare, excepting 
those which either live in or burrow into the coral rocks or sand. The species of 
Donax, so common at Mayaguez, is an excellent illustration of this, and Cytherea 
dione , found only at Fajardo, is another. Starfishes and sea-urchins, except those 
species which live in or under rocks or among the branching corals, are very rare, and 
such other animals as lie exposed upon the bottom, or which have no special means 
of maintaining themselves, were poorly represented or entirely absent. The facies of 
the fish-fauna has been determined in the same way. Brackish -water, free-swimming, 
shallow- water, and surface-swimming fishes are notably absent, while there are many 
blennies and gobies living in, under, or among the rocks and in the reefs; gobies, 
chietodonts, and the like, in the tide-pools or in holes in the rocks along the shore, 
and scaroids and blennies among the algae. Among free-swimming fishes as a rule 
only those species are well represented which live at sufficient depths to prevent their 
being seriously disturbed by the constant swashing along the shore. In short, all the 
shore species, not only of fishes but of all other groups, are those which have been 
able to maintain themselves either by holding to something, by burrowing or crawling 
into the rocks, by living in protected nooks and corners along the shore, by living in 
patches of algae, by burrowing in the sand, or by darting into protected places when 
the surf becomes too strong. 
The peculiar physical conditions described above also account for the scarcity of 
marine mammals and reptiles, and for the apparent absence of commercial sponges 
about the island. The only marine mammal known from Porto Rico is the manatee 
(probably Trichechus latirostris ), and it is of very rare occurrence, owing no doubt, 
to the absence of broad sluggish rivers in which it finds its favorite environment. 
Turtles are also uncommon. The species represented are said to be the hawksbill 
(Eretmochelys imbi'icata) and green turtle ( Ghelonia mydas), which are rare, except 
at the east end. The scarcity of turtles is doubtless due to the absence of large areas 
of shallow water with sandy bottom. So far as known, there is no species of alligator, 
crocodile, or seal about this island. 
What has been said of the marine fauna of Porto Rico applies equally well to the 
inhabitants of the streams of the islands. In the rivers are found more than a dozen 
species of fishes, most of which are strictly fresh-water species or fishes which run well 
up fresh-water streams, and with these occur several species of shrimps and prawns. 
All the rivers of Porto Rico, as already stated, are swift, turbulent streams at all times, 
and during heavy rains they become veritable torrents, carrying everything caught in 
the current far out to sea. Fresh-water inhabitants of these streams, in order to 
escape being swept into an adverse environment in the sea, have acquired the habit of 
burrowing or going into holes in the banks where they are comparatively safe, even 
during the greatest of floods. During the evolution of this habit the individuals 
which tended to seek the holes in the banks most promptly, and to remain in them 
most persistently, would stand the best chance of surviving, and the result in time has 
been species that habitually stay in the protected shelters or which do not wander far 
away. 
