SCELOPORUS. 63 
the species of a certain comparatively limited district are under examination; then I 
have always found that no, or but little, variation obtains among the specimens. Thus 
I have seen from fifty to 100 examples of a species from the same locality, all agreeing 
in the form of the supraoculars, length of limbs and toes, size of scales, prominent 
markings, and showing variations only in the irregular mottling, or in the shade of the 
ground-colour, and in the number of femoral pores. 
The difficulties are greatest among the large-scaled forms, that is among those which 
have some twenty-two to thirty-five scales in a line between the collar and beginning 
of the tail; and these difficulties have been increased by the attempts to unite as many 
modifications as possible under the same specific term. Consistently, either they all 
must be thrown together, or many more forms should be distinguished than have 
been admitted hitherto. Species described as distinct, and from distant localities and 
different altitudes, should not be thrown together without direct evidence of their 
absolute identity. 
Our collection, like that of every other museum, and the information as to altitude 
of the specimens, is much too incomplete to give anything like a satisfactory account 
of the Central-American species of this genus. But, by testing their value by an 
examination of several series, each from the same locality, I come to the conclusion 
that the following characters can be relied on :— 
1. The size of the scales. 
2. The arrangement of the large supraoculars. 
3. The length of the hind limbs and hind toes. 
4, The presence or absence of the nuchal collar, and its extent. 
5. The markings, if any, in front and behind the shoulder-joint. 
6. The coloration of the abdomen of the adult male. 
A. The scales on the sides gradually passing into, and not conspicuously 
larger than, the dorsal scales. 
a. No preanal pores. 
1. Sceloporus spinosus. 
Sceloporus spinosus, Wiegm. Isis, 1828, p. 370; and Herp. Mex. p. 50, t. 7. fig. 3; Baird, U.S. & 
Mex. Bound. Surv., Rept. p. 5, t. 29. figg. 4-6; Bocourt, Miss. Sc. Mex., Rept. p. 174, 
t. 18. figg. 2, 2a, 6; Cope, Proc. Am. Phil. Soc. 1885, p. 399; Boul. Cat. Liz. ii. p. 226. 
Sceloporus horridus, Wiegm. Herp. Mex. p. 50; Bocourt, Miss. Sc. Mex., Rept. p. 178, t. 18. 
figg. 8,8 a,b; Cope, Proc. Am. Phil. Soc. 1885, p. 396. 
Sceloporus clarkii, Baird & Gir. Proc. Ac. N. Se. Phil. vi. p. 127; Yarrow, in Wheeler’s Report 
Explor. W. 100th Merid. v. p. 575, t. 23. figg. 1, la. 
Sceloporus magister, Hallow. Proc. Ac. N. Se. Phil. vii. p. 93. 
Sceloporus oligoporus, Cope, Proc. Ac. N. Sc. Phil. xvi. p. 177. 
Hab. Norra America, Southern States.—Mexico, Sonora (Hallowell). 
