420 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
evolutionary point of view. Amerva vittipunctata in size, in certain 
color-pattern features, and in many details of scutation, is similar to 
A. erythrocephala; a species with which it doubtless has but a rather 
distant relationship. Ameiva exul has its nostril between the two 
nasal plates, a character typical of the mainland and southern Lesser 
Antillean species, but otherwise it is not anomalous. The characters 
which in general we have found to be most constant in species of this 
genus are to be seen among the supraoculars, gulars, antebrachials, | 
brachials, postbrachials, ventrals, and tibials. 
In view of this variability noticeable in the island, and greatly 
exaggerated in the mainland, forms, we must either recognize a number 
of subspecies or merge all of the mainland races into five or six species. 
To do this, especially since we find that some variations have a definite 
relationship to their distribution, would be to obscure the true state 
of affairs, especially since we find that in some of these races speciation. 
has far advanced and the appearance of any barrier to an interchange __ 
of individuals would doubtless result in the fixation of a valid species — 
ina short time. We therefore recognize several subspecies of Ameiva — 
ametva, two of A. undulata, and one of A. bifrontata. 
The whole question of explaining the origin of this genus and its — 
dispersal is difficult and unsatisfactory. We may say, fairly that 
Ameiva and its possible offshoot Cnemidophorus represent the most — 
generalized, perhaps the most primitive existing representatives of the © 
characteristic American family Tetidae. Of the geologic history of — 
this family we know really nothing; we can only postulate its origin © 
by saying that along with the much more archaic Xantusiidae the — 
Teiidae probably arose in America from early immigrants of the same — 
stock which in the old world has given rise to the Varanidae. ‘That — 
this migration took place from eastern Asia to America by way of the — 
Bering Strait land bridge is not improbable. Change of climate 
then probably forced the ancestral teiids southward and they flourished — 
and are now wholly confined to the tropics, except Cnemidophorus 
sexlineatus, which has invaded secondarily the Austroriparian zone of - 
North America, and a few which have pushed into temperate South 
America. Our study leads to the conclusion that the existing Ameivas 
have not all arisen in one region as Gadow shows was most probable © 
for the Cnemidophori, but rather that they have probably spread from 
two centres. We submit then that probably some widespread ances-— 
tral Ameiva-like stock left two relict types, one of which gave rise to 
A. undulata and its allies, and the other A. ameiva and its relatives. 
The difficulty with this explanation is the fact that part of Central 
