MEXICAN SNAKES AND CftOCODILIANS — SMITH 399 



Each species has about the same number of lateral spots ; in nummifer 

 the dorsal blotches are always of about the same number as the lateral 

 spots ; but in mexicanus the number may be reduced to as few as 10 or 

 11 by fusion of pairs of blotches. 



Other pattern differences involve the tail and head. In mexicanus 

 the tail tip is very light (yellowish) in young specimens, and it 

 remains light until a size of at least 500 mm. is reached; specimens 

 of greater size than this may develop dark color at the tip of the 

 tail or may retain a light color. In nummifer the tail tip may be 

 slightly lighter than the base, but it is well pigmented in even the 

 youngest specimens; never is it distinctly light as in mexicanus. In 

 adult nummifer the tail may remain dark or become rather light at 

 the tip. 



The position of the postocular dark stripe is generally diagnostic. 

 In mexicanus it continues straight back from the eye, parallel to the 

 lip, for about one-third its length; it then dips ventrally and reaches 

 the level of the mouth just posterior to the rictus oris, terminating 

 three or four scales posterior to that point; it usually involves only 

 the posterior edge of the last scale in the lower row of temporals and 

 never involves more than the last two scales. In nummifer the stripe 

 passes almost in a straight line diagonally toward the angle of the 

 mouth, thus almost always involving at least two, and usually three 

 and sometimes four, of the scales in the lower row of temporals. 



There are apparently no differences in ventral pattern. 



In scutellation there is a difference in number of scale rows, which 

 is 23 to 27 (one 23 in 6) anteriorly and 25 to 27 medially in mexicanus, 

 but 21 to 25 anteriorly (one 25 in 13) and 23 to 25 medially (one 25 

 in 15) in nummifer. Ventrals in two male mexicanus are 124 to 127, 

 in eight male nummifer 129 to 134; in four female mexicanus they 

 are 127 to 131, in six female nummifer 120 to 134. Caudals in two 

 male mexicanus are 32 to 36, in nine male nummifer 30 to 36 ; in four 

 female mexicanus they are 31 to 35, in six female nummifer 23 to 30. 

 Thus differences are apparent between the two forms in the ventrals 

 of males and the caudals of females. In mexicanus the supralabials 

 average somewhat more numerous, 9 seldom occurring (1 in 12 

 counts), and 11 frequently occurring (5 in 12 counts), while in num- 

 mifer 8 or 9 frequently occur (19 in 30 counts) and 11 not at all. In 

 mexicanus the nasal is completely separated from the rostral by three 

 small scales, while in nummifer it contacts the rostral, and the row 

 of small scales is reduced to two, one, or none. The temporal scales of 

 the lower row are somewhat smaller in mexicanus than in nummifer. 

 In mexicanus the lower preocular is very poorly differentiated and 

 does not enter the orbit; in nummifer it enters the orbit below the 

 much larger upper preocular and is rather well defined. 



