1906.] IN MEXICAN LIZARDS. . 363 



brown fields. The mid-field between the narrow stripes 3-3 is 

 pale, bordered by rows of dark specks. 



Stage B. — Faint, pale brown spots appear in the first and second 

 fields, and the mid-field becomes lighter in this way that the dark- 

 pigment is arranged in more continuous lines against the inner 

 borders of the third strij^es ; and occasionally there appears a 

 darker central streak in the broadening mid-field. Then, with a 

 length of about 70 mm., the field-spots, which are never sharp, 

 become lighter and more numerous, and arrange themselves in 

 one or two rows in each field, and the pale portions of the 

 widening mid-field become greenish. 



Stage 0. — When the lizards approach maturity, length about 

 100 mm., the stripes 1 and 2, hitherto very conspicuous, become 

 dull and lose their sharp contours. The pale field-spots become 

 transveisely confluent Avhere they existed in double rows in a 

 field, or they become enlarged transversely, so that each field is 

 broken up into some 20 or more dark cross-bars, alternating with 

 pale bars. Both kinds of bars encroach upon the dissolving- 

 stripes 1, 2, and 3, whilst the remaining portions of these lines 

 join, or merge into, the pale brown or olive-grey, which gradually 

 becomes the predominant ground-colour. 



Stage D. — Ultimately the whole back and the sides of the body 

 assume a veiy complex pattern : brown, pale brown, olive, and 

 whitish colours, mottled or vemiculated ; on the whole, however, 

 decidedly cross-barred. The black bars are of course most 

 consj)icuous, and in some cases the black bars of the right and 

 left sides meet across the back, producing a strikingly handsome 

 tiger-pattern. The extent to which the longitudinal stripes 

 disappear varies much, and in the adult of both sexes the detail 

 of the whole complicated pattern is scarcely the same in two 

 individuals from the same locality. 



A noteworthy character of these lizards is the complete absence 

 of any pale spots except those transitory faint spots in the fields of 

 young specimens. In this respect they difier conspicuously from 

 C. communis and its relations, with their numerous sharply marked 

 white, yellow, or blue spots either all oA'er the upper surface, or at 

 least on the rump, root of the tail, and on the thighs. The thighs 

 of sj)ecimens from Oaxaca and Totolapan are always marbled, 

 and the usual white stripe on the posterior side of the thigh is 

 broken up and disappears at an early stage. 



Cnemidophorus mexicanus, var. balsas. (Text-fig. 83.) 



Number of specimens examined about 71. 



Within the Basin of the Balsas River, from Cuernavaca in the 

 north to Chilpancingo in the south, the genus Cnemidojjhorus is, 

 besides C. deppei^ represented by a form which difFei'S from the 

 typical C. communis occidentalis mainly in the evolution of the 

 dorsal pattern. It might be described as an intensified, enlai'ged 

 C. gularis of which the stiipes become destroyed by invasion from 

 the fields, whilst they are not broken up into series of light spots, 



