158 BULLETIN Gl, UNITED STATES NATIONAL MUSEUM. 



coldness of the climate will permit; (4) the smallest size and scu- 

 tellation is fomid in the form tliat constitutes the distal end of 

 the line of genetically related forms; (5) the intermediate region 

 between the forms is narrow in every case, showing that the change 

 in scutellation takes place- rapidly. Again, if the relationships are 

 as I have pointed out, there is a remarkable case of convergent 

 evolution in this group. This is shown by the great similarity in 

 scutellation between scalaris and ordinoides. If it were not for the 

 peculiar color pattern of the former, it would be imp()ssi]:)le without 

 a knowledge of the locality to distinguish specimens of these forms. 



ordinoides 



elegans 



\ 



hammondi 



angustirostris 



melanoffaster 



scalaris 

 phenax 



Fig. 69.— Fhylogenetic dkvelopment of the Eleg.\ns group. 

 THE SIRTALIS GROUP (EQUES, SUMICHRASTI, PARIETALIS. SIRTALIS). 



EQ UESa' 



Description. — As in the case of all of the other groups, the maximum 

 scutellation for the Sirtalis group (as I have defined it) is exhibited by 

 a species which has its principal distribution on the Mexican plateau. 

 This form is the Coluber eques of Reuss, the Eutaenia cyrtopsis of Ken- 



a ThamnopMs eqiies (Rev ss). Mus. Senckenb., I, 1834, pp. 152-155. Includes E. 

 cyrtopsis Kennicott, Thamnophis cyrtopsis cyclidcs Cope, Tropidonotus collaris Jan, E. 

 cyrtopsis ocellata Cope, E. eques am-ata Cope, E. piilehnlaius Cope, E. dorsalis Baird 

 and GiRARD, and E. ornata Baird and Girard. 



