Geometrica-Group of S.A. Tortoises. 183 



black, while the sides are a light yellow, the boundary between the 

 two being sharply defined. In some specimens from the sanie sub- 

 group the beginnings of a breaking up of the central patch into 

 separate radii are represented, and triangular rays also begin to 

 appear on the yellow sides. The latter variation is well marked in 

 specimens of tentoria from the Hex River Valley. The same stage is 

 also characteristic of trimeni, an excellent representation of which 

 is given by Boulenger, 1886, pi. LVII. In geometrica the middle 

 patch is likewise partly broken up into definite rays, and rays are also 

 developed along the sides. Most of the rays in these instances are 

 densest at the margin of the plastral shields, and start from the an- 

 terior suture, though some originate from the middle suture. A close 

 comparison shows that the rays in the different types correspond with 

 one another, ray for ray. 



In the verreauxii sub-group the plastron is at times only a faint, 

 dirty brown, perhaps a little darker in the middle. This variation 

 is represented by Boulenger in his figure of T. fiskii, 1886, pi. 

 LVIII. Frequently, however, in verreauxii the plastron is strongly 

 rayed with black or dark brown in a regular fashion, the individual 

 rays corresponding with those of the forms just mentioned. Boulen- 

 ger's figure of T. smithii shows this stage (B.M.C., pi. IV.), while 

 that of T. seimimdi (1903, pi. XVII.) displays it in an incipient 

 condition. So far, then, as the plastron in verreauxii is rayed, the 

 arrangement of the rays is exactly like that in other sub-groups. 



The number of the dark brown rays on the plastral shields, and 

 the complexity of the pattern produced thereby, reach their climax 

 in the oculifera sub-group. Here the whole plastron has a yellow 

 background on which are dark brown or black rays, arranged in a 

 more or less regular manner, so as to produce a geometrical pattern. 

 Each specimen presents a similar plan, with many variations in such 

 details as the width of the rays, their number, and curvature. But 

 each of the principal rays has its exact counterpart in verreauxii 

 and the others ; the number of rays may be increased by the partial 

 or complete splitting of individual rays, but not by the intercalation 

 of any new rays. The extreme rayed condition displayed in oculifera 

 is foreshadowed in a complete gradational series, beginning with 

 the uniform black and yellow of certain tentoria. 



Thus, as regards the coloration of the plastron, we reach the 

 same results as from a study of the coloration of the carapace. 

 Starting with the simple condition found in certain members of the 

 tentoria sub-group, we obtain a transitional series of variations lead- 

 ing to the very complex condition characteristic of the oculifera sub- 

 group. Exactly corresponding rays can be traced in both carapace 

 and plastron from an incipient stage to a very complex pattern of 

 the same elements. In the plastron, however, we commence with a 

 single uniformly black patch, and then trace its differentiation into 

 separate rays, accompanied by the appearance of new rays on the 

 sides, while in the carapace no new elements are introduced at any 

 stage. 



