OCR Text |
Show 338 DR. II. GADOW ON EVOLUTION [Mar. 20, one complete row of large scales which form the edge ; upon this follow, towards the throat, several shorter rows of scales which decrease in size. Posterior surface of forearm with at least some large scutes. Frenocular variable. The young start with from 6 to 8 whitish stripes, which become dull, whilst white spots develop within most of these stripes. Fields at first dark, later on light spots develop in them, mostly rounded and well-defined. Ultimate result: many spots on very dark ground in about 10 longitudinal rows, and numerous small whitish spots on the rump, root of tail, and on the thighs. Throat and collar light-coloured, often pink. Chest and abdomen are early suffused with blue; with advancing age chequered blue and black, with whitish edges to the scales. Cope was quite justified in separating Mexican Cnemidophori of larger size, with essential gularis structure (4 supraoculars, sti'ong collar, and large forearm scutes), and in which the stripes break up into rows of spots, as C. gularis communis ; but he did not know, or he ignored, C. bocourti, and he had only a very insufficient Mexican material. The diagnosis or description given above suits the majority of those Cnemidop>hori which are known from the western half of the Mexican plateau and its western and south-western slopes, from the north-west of Chihuahua to Colima and Manzanillo ; and across the plateau from, roughly speaking, Guadalajara, to Guanajuato and Puebla. But in this wide stretch of varied country they exhibit considerable changes,-changes which at first crop up as unimportant, individual variations, but which in neighbouring districts have become the rule; and to these are added changes of other characters, until their combination completely upsets the original diagnosis. Thus, for instance, in Michoacan the stripes are more persistent and the scutes of the forearm are more polygonal, smaller, even reduced to granules. In Colima, the pores and the rows of scales on the humerus and femur are distinctly more numerous. At Manzanillo, these changes are combined with smaller collar-scales ; while on the Isthmus of Tehuantepec and in Oaxaca, at Cuicatlan, an entirely granular forearm is added; so that nothing is left which could justify us to enumerate these specimens as a subspecies or a race of C. gularis, whilst they could well figure as a race of C. communis. At the same time, they approach the less typical specimens of C. immutabilis and C. guttatus to such an extent, that it is not always easy to keep them asunder. Further, in the basin of the Balsas River C. communis is represented by a form which is structurally an intensified C. gularis, and removed as far as possible from the southern variations, but the spotty character is gone, and the tendency to destroy the stripes by cross-bars begins to assert itself, until further east, in Oaxaca, the old specimens are tiger-barred with a variable, partly granular collar and with smaller and fewer scutes on the forearm. These are C. mexicanus, which may well |