THE SNAKE GENUS TRIMORPHODON — SMITH 107 



color is not so abruptly truncate posteriorly, terminating posterior 

 to the parietals; and the light, midparietal indentation of tau is re- 

 placed by a narrow, Y-shaped mark, the arms of which follow near 

 the posterior sutures of the frontal, and sometimes reach to the outer 

 edge of the supraoculars, where they join with the tips of the inter- 

 ocular light bar. 



Variation in body pattern in tau is so great that no contrast of the 

 species as a whole with upsilon is possible. The variants of tau ap- 

 pear to be segregated geographically but are represented by so few 

 specimens that the apparent differential characters of the three popu- 

 lations indicated may not be well founded. 



The range of tau is apparently the periphery of the central Mexican 

 plateau. The extreme southern records near Oaxaca city, in the iso- 

 lated mountains of central Guerrero, and in the mountains at the ex- 

 treme edge of the plateau in Michoacan all indicate such a peripheral 

 distribution. All three loci represented by specimens, however, are so 

 far removed from each other that the peculiarities of each popula- 

 tion (two of which are represented by single specimens) may prove 

 to have special significance: that is, at least three subspecies may 

 exist in tau: 



1. Oaxaca specimens (3). Dorsal blotches 23 to 26; tail bands 

 9 to 10; belly very little pigmented; subcaudal surface nearly uniform 

 white; interocular band complete; nuchal blotch two to three scale 

 lengths behind parietal; body blotches (except two nuchal ones) in- 

 volving three or fewer scales in first row, average two. 



2. GuERRiTvO specimen CI). Dorsal body blotches 22 ; tail blotches 

 8; belly heavily pigmented, the dorsal bands visible (not sharply 

 defined) ; subcaudal surface very strongly mottled ; interocular band 

 reduced to a round spot in middle of frontal; nuchal blotch five 

 scale lengths behind parietal; body blotches Cexcept two nuchal) 

 involving two to six scalas in outer row, average five. 



3. Michoacan specimen (1). Dorsal body blotches 34; tail bands 

 11; belly with some dark spots, poorly defined; subcaudal surface 

 moderately pigmented ; interocular band complete ; nuchal blotch one 

 scale length behind parietal; body blotches not well defined on outer 

 scale rows, involving two or three scales on outer row where visible. 



TREMOEPHODON VILKIN'SOril Cop« 



Trimorphodon vUkinaonii Cope, 'Pt()C. Amer. Philos. SfK-., vol. 23, pp. 285-286, 

 1886 (Chihuahua).— TAYfXB, Kan.«a.s Univ. Sci. Bull., vol. 25, pp. 361-36.S, 

 fig. 1, pi. 38, 19.38 (1939).— Kr^uEEE, Trans. San Diego Soc. Nat. HLst., vol. 9, 

 pp. 187-189, 1940. 



Diagnosis. — ^A broad, light area on neck, between dark areas on 

 head and first body blotch; bands a third length of interspaces; dark 

 head area only three spots in yoiing. 



