256 BULLETIN 15 4, UXITED STATES NATIONAL MUSEUM 



the northern Cnemidopliori. In fact it exhibits a marked simiLarity 

 to this southern, wide-ran<>ino:, plastic form, differing chiefly in the 

 absence of one of the supraocular shields, in the loss of the outer 

 pair of large parietal plates and in the changed position of the 

 nasal opening. It may be remarked here that the last two of these 

 modifications are retained in the remaining groups of Cnemido- 

 pJiorus^ in -which, in addition, other changes have occurred. In 

 deppii and Jemniscatus the distinctive coloration and general bodily 

 size of the young are practically identical and even the adults often 

 resemble each other in manj^ Avays. 



The deppii protostock was probably not evolved before the late 

 Miocene. If it had existed earlier it would probably have spread 

 to the Greater Antillies, but deppii has not been found there. In 

 fact, it is even absent from old Providence Island where leniniscatus 

 now occurs. After its evolution the ancestral deppii stock was ap- 

 parently divided into two parts by the opening of a marine portal 

 across the Isthmus of Tehuantepec. Of the time of formation of 

 this barrier to the nortliAvard migration of southern elements, 

 Schuchert has said (1929, p. 141) that "it was certainly open dur- 

 ing early Pliocene time, and probably also during the late Miocene."' 

 The southern section of the isolated ancestral deppii stock probably 

 gave rise to the modern deppii^ while the isolated northern section 

 changed to produce the forerunner of the present Mexican guttatus. 

 After the closure of the Tehuantepec portal the hardy deppii stock 

 again migrated northward. Thus it was brought into the area now 

 occupied by guttatus. Apparently in recent geological times, prob- 

 ably in the late Pleistocene, a small portion of the population of 

 deppii, isolated on islands off the coast of Quintana Roo, Peninsula 

 of Yucatan, has changed to cozumelm, a closely allied subspecies of 

 the former lizard. 



Because of the general retention of four supraoculars in the higher 

 groups of Cnemidophorus^^ it is improbable that their progenitor 

 arose from a completely differentiated deppii stock. However, be- 

 cause of the dark blue ventral suffusion found in both the modern 

 gularis and the modern deppii and the possession of other fea- 

 tures in common, a mutual origin from a Icmniscatu8-\\V^ stock is 

 suggested. 



As to the method of derivation of the higher groups of Cnemido- 

 phorus from tliis lemniscatus-likQ stock, it is known that the geology 

 of the Central American region has been much varied, and there- 

 fore, there was probably ample opportunity for the ancient isolation 

 of a part of the southern lemniscatus stock to produce the ancestral 



^^ It may be noted here tliat deppii itself occasionally retains the original four supra- 

 oculars (see p. GO for details). 



