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Show 1906.] IX MEXICAN LIZARDS. 293 roughly speaking within a triangle from Tampico and Vera, Cruz to Zacatecas. Most of the latter State lies too high ; and this altitude would be a sufficient factor for stopping the eastward extension of C. communis and its allies. No Cnemidophori have ever been recorded from Jalapa, although that district has been the hunting-ground of many good collectors ; none are known from Orizaba district, and the Oomision cientifica (cf. Cope's List, Proc. A m . Phil. Soc. 1885, p. 372) returned none from the State of Hidalgo. The northern half of the State of Vera Cruz is covered mostly with rain-forest. The reputed absence in the triangle is easiest accounted for by the assumption that C. gularis coming from the North, and C. deppei with C. guttatus from the South, have not yet met, perhaps cannot meet on account of unsuitable bionomic conditions. Evolution of Pattern of C. tessellatus A - D and of C. rubidus E. How, then, have we to imagine that the spreading of Cnemidophorus in Mexico has taken place ? Of course we leave aside the idea of a multiple origin. The usual explanation of zoogeographers would be as follows :-Some indifferent species spreading from the South through the Pacific half of the country northwards, and thence into the United States, has on its way given rise to the various forms of lizards. This not unreasonable assumption, if applied to the species as we actually find them distributed, would imply that they have changed, say, from A into B into . . . E, each with side-branches or sub-species, but that on their arrival in the North in the form E they have been turned again into something like A. Deppei and sexlineatus are near allies, but such a reversion or return to pristine conditions is most unlikely. It would, moreover, mean that E, while assuming ^4-like characters, must also be rather like B. But in reality this is not the case. On the contrary, A [sexlineatus) turns into E (gularis); this in tarn into I) and C {communis occidentalis); C into B (C. copei), the var. australis of which we have some occasional difficulty in 20* |