212 BULLETIN 114, UNITED STATES NATIONAL MUSEUM. 



tendencies expressed as individual variations in amaura and other 

 forms of the triangulum group. Following is a summary of the most 

 evident differences: 



Decrease in bodily size has resulted in (1) a reduction in number 

 of ventral plates — a change that is paralleled by several other forms 

 in the group and genus, the only difference being that here the de- 

 crease is more pronounced; (2) a slight decrease in number of caudal 

 scutes, resulting in a slightly greater proportionate tail length; (3) 

 a decrease in number of dorsal scale rows to a point that averages 

 even lower than the lov/est individual variation in other members 

 of the group and genus. Accompanying decrease in bodily size the 

 head has become smaller and slightly different in shape; (4) the lower 

 labials are reduced by one in the majority of individuals; (5) the 

 temporals are reduced by one in each row; (6) the loreal has become 

 narrow and elongate, and in many individuals it has been replaced 

 entirely by a downward extension of the prefrontal plate; (7) the 

 supraoculars have become small; (8) the rostral has become more 

 pointed and slightly extended beyond the lower jaw. The pattern 

 has suffered less marked change; (9) the paired black annuli have 

 been reduced by about two in correlation with the change in number 

 of ventrals and caudals; (10) the ringed pattern has become well 

 fixed and there is less widening of the yellow annuli on the sides; 

 (11) the black pigment has receded entirely from the snout, and in 

 most cases has become restricted to the posterior portion of the head. 



That two forms, directly related, can be so distinct at the common 

 boundary of their ranges calls for an explanation. First, it must be 

 recognized that elapsoides is more profoundly modified than any 

 other member of the group. This undoubtedly means that it is 

 older than the others, or that it has been isolated for a considerable 

 time, or both. If we assume a northeastward migration from 

 northern Mexico antedating that by which triangulum, syspila, and 

 the others may be supposed to have reached the United States, we 

 have an explanation for elapsoides. If, after this form or its fore- 

 runner reached the southeast, an emba3^ment from the Gulf of 

 Mexico extended up the lower Mississippi Valley (and it is probable 

 that such actually occurred during the Quaternary), it might have 

 divided the ancestral stock into an east and a west division. The 

 section in the southeast might then very conceivably have undergone 

 a modification, due to its isolation, into what we now know as elap- 

 soides, while the main stock, still connected with its Mexican rela- 

 tives, was but slightly changed. When the gulf waters receded the 

 ranges of the isolated sections would have been joined again. It 

 then appeared that differentiation had been so great that intergrada- 

 tion seldom if ever occurred, while it was not sufficiently great to 

 allow either form to encroach much upon the range of the other. 



