184 Report S.A.A. Advancement of Science. 



As regards the coloration of both the carapace and plastron, no 

 additional type of variation is anywhere introduced. We have cor- 

 responding series of rays in each, and evolution consists in the 

 elaboration of features common to all at one stage or another. The 

 passage from one extreme to the other is by direct and gradual varia- 

 tion. The great variation in details shown in different individuals 

 from the same locality indicates that the plan of coloration is not 

 yet wholly fixed and permanent in any sub-group, though within 

 varietal limitations it can be considered as characteristic. 



c. Axillary and Inguinal Shields. 



Siebenrock, in his paper, " Die siidafrikanischen Testudo-Arten 

 der Geometrica-Gruppe s.l.," has established as a character of some 

 diagnostic value within the geo metric a-gxowp the occurrence of either 

 one or two axillary shields ; in fact, in his Synopsis of the species, he 

 employs it as a means for separating the members of the group into 

 two series. The first series includes geometrica and oculifera, both of 

 which have only one axillary shield on each side, and the second all 

 the remaining species, in which the axillary shield is divided into two 

 small shields. The distinction holds as a general rule for all the 

 specimens I have examined, but the acceptance of the character 

 must not be taken as necessarily implying the direct relationship of 

 the forms presenting the one or the other condition, for in nearly all 

 other respects geometrica and oculifera are far apart. 



The actual condition of the axillary shield within the geometrica- 

 group is not so sharply defined as Siebenrock's use of it would imply. 

 Both the axilliary and the inguinal shields represent the last remains 

 of a series of numerous plates which in some Chelonia are inter- 

 calated on the bridge between the marginals and plastrals, and are 

 known as inframarginals. Different members of the geometrica- 

 group are found to vary as to the degree to which the reduction in 

 number of these inframarginals has taken place. Both geometrica and 

 oculifera, which in some other respects represent extreme types of 

 variation, have the axillary shields reduced to one, and in nearly 

 all the specimens of oculifera examined by me the shield is united 

 with the femoral without any hint of a suture. In the other members 

 of the group the reduction has not been carried so far, and a longi- 

 tudinal series of two, three, or even four or five inframarginal shields 

 may occur, which diminish in size in passing forwards. Individuals 

 of tentoria generally show more than two shields ; three or four are 

 usually present, and occasionally one or two more, though not always 

 in the same straight series. In verreauxii there are rarely three, usually 

 there are two, the posterior somewhat larger than the anterior, and 

 occasionally there is only one, while, as we have already seen, the 

 latter condition is characteristic of geometrica and oculifera. Occa- 

 sionally in verreauxii the number on one side is different from the 

 number on the other. 



Thus within the geometrica-group we have all variations repre- 

 sented in the reduction of the number of anterior inframarginals 



