420 bulletin: museum of comparative zoology. 



evolutionary point of view. Ameiva 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 

 in a short time. We therefore recognize several subspecies of Ameiva 

 ameiva, 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 Teiidae. 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 diflliculty with this explanation is the fact that part of Central 



