THE SNAKE GENUS TRIMORPHODON — SMITH 



153 



Jeast suggests the possibility that vUkinsoniih pattern was produced 

 in the same manner. The multiplicity of blotches in certain central 

 (Guanajuato?) specimens of upsilon is an apparent step in this di- 

 rection. Possibly specimens from areas between Zacatecas and Chi- 

 huahua would show whether such a course may have been pursued 

 in the evolution of v'dkiiisonii. 



In view of the fact that several morphological changes took place 

 in the hiscutatiis group, with differences apparent in subcaudals, 

 hemipenis, and anal plate, it is remarkable that only one species in 

 the upsilon group possesses morphological characters sufficiently dif- 



FiGURE 38. — Distribution of the species of Trimorphodon. Inverted triangles, iau; triangles 

 not inverted, b. biscutatus (unless otherwise indicated); dots, unless otherwise indicated, 

 upsilon; vertical cross hatching, lambda; horizontal cross hatching, lyrophanes; diagonal 

 cross hatching, vandenburghi. 



ferent from the group norm to identify it. This species {forhesi) 

 is very much like upsilon in pattern, and its apparently recent devel- 

 opment tempts a chronological association with the development of 

 the species in the other group with a single anal {vandenburghi). 



With respect to pattern, it is noteworthy that, curiously, the end 

 form in neither group has undergone sufficient morphological dif- 

 ferentiation that it may tliereby be distinguished from the members 

 of the group to which it belongs. 



The relative age of the two groups is difficult to determine. One 

 group {tiscutatus) appears to be of lowland origin, while the other 



