CHAP. XIV.] 
THE NEOTROPICAL REGION. 
59 
by one or more arms of the sea, as above indicated, is further 
rendered necessary by the character of the molluscan fauna of 
the Pacific shores of tropical America, which is much more 
closely allied to that of the Caribbean sea, and even of West 
Africa, than to that of the Pacific islands. The families and 
many of the genera are the same, and a certain proportion of 
very closely allied or identical species, shows that the union of 
the two oceans continued into late Tertiary times. When the 
evidence of both land and sea animals support each other as 
they do here, the conclusions arrived at are almost as certain as 
if we had (as we no doubt some day shall have) geological proof 
of these successive subsidences. 
Islands of the Mexican Sub-region. — The only islands of 
interest belonging to this sub-region, are Tres Marias and 
Socorro, recently investigated by Col. Grayson for some of the 
American Natural History societies. 
Tres Marias consist of four small islands lying off the coast 
of north-western Mexico, about 70 miles from San Bias. The 
largest is about 15 miles long by 10 wide. They are of horizon- 
tally stratified deposits, of moderate height and flat-topped, and 
everywhere covered with luxuriant virgin forests. They appear 
to lie within the 100 fathom line of soundings. Fifty-two species 
of birds, of which 45 were land-birds, were collected on these 
islands. They consisted of 19 Passeres; 11 Picariae (7 being 
humming-birds) ; 10 Accipitres ; 2 parrots, and 3 pigeons. All 
were Mexican species except 4, which were new, and presumably 
peculiar to the islands, and one tolerably marked variety. The 
new species belong to the following genera; — Panda and 
Gmnatellus (Mniotiltidie) ; Icterus (Icteridse); and Amazilia 
(Trochilidas). A small Psittacida differs somewhat from the 
same species on the mainland. 
There are a few mammalia on the islands ; a rabbit (Lepus) 
supposed to be new ; a very small opossum ( Didelphys ), and a 
racoon (Proeyon). There are also several tree-snakes, a Boa , and 
many lizards. The occurrence of so many mammalia and snakes 
is a proof that these islands have been once joined to the main- 
land ; but the fact that some of the species of both birds and 
