4 PROCEEDINGS OF THE NATIONAL MUSEUM vol.73 



The presence of several species within a short radius makes it im- 

 possible to determine the origin of the various species. The order of 

 arrangement of the species in this paper has been made with regard 

 to structural relationship and not according to geographical distribu- 

 tion or to any supposed origin. 



During the lower and upper Cretaceous, the upper Eocene and the 

 early Oligocene periods what is now the peninsula of Lower California 

 was a part of the mainland of Mexico, the present Gulf of California 

 being dry land. Some of the members of this tribe of lizards migrated 

 northward and westward, away from the foothills of the mountain 

 ranges, finding their way to the semiarid desert regions of the Pacific 

 coast, what is now the Cape St. Lucas region of Lower California. 

 During the late Oligocene period the land between the desert region 

 and the mountain foothills became submerged, creating the present 

 Gulf of California. The ctenosaurs that were then shut off from 

 their kindred on the mainland became adapted to the deserts, under- 

 going of course a few minor changes such as would aid in the preser- 

 vation of the species. The chief changes were in the shortening of 

 the dorsal crest, both in the length of the individual spines and also 

 in the extent of the crest on the back. Color markings were effected 

 to give greater protective resemblance; resemblance to the speckled 

 and splotched habitat of the species. This species is called Ctenosaura 

 Jiemilopha. Its present range is the entire southern half of Lower 

 California and most of the islands near the peninsula, in the Gulf of 

 California. A few individuals have been collected just across the 

 gulf in Sonora, and as far north as Nogales, Ariz. They were in all 

 probability carried there by man; but it is not impossible that their 

 ancestral stock wandered there before the submergence of the Gulf of 

 California. 



In the immediate vicinity of the center of distribution of the genus 

 four species have arisen. They probably arose in the following order: 

 hrachylopJia, brevirostris , pectinata, and parlceri. As the original stock, 

 acanthura, continued its migration southward, other species appeared; 

 clarJci and quinquecarinata. South of the Isthmus of Tehuantepec 

 acanthura is replaced entirely by similis, a very active form which is 

 abundant throughout Central America as far south as Panama. As 

 the lizards continued thek southward migration, new conditions in 

 their surroundings led to new structural adaptions. A change in color 

 took place, transverse stripes becoming conspicuous, and these prob- 

 ably serve, as in the case of the tiger, to aid in the concealment of their 

 possessor. This coloration is associated with changes in habits in 

 Central America. Three other smaller species, halceri, palearis, and 

 defensor, each with a very restricted habitat, have also arisen in this 

 territory, all coming perhaps from similis; halceri is restricted to Utilla 

 Island, Honduras, defensor to northern Yucatan, and palearis to the 



