Geometrica-Group of S.A. Tortoises. 187 



An extreme variation is reached in some representatives of the 

 geometrica sub-group ; the shield has become very narrow, two or three 

 times longer than broad, and pointed at its free end. In others of 

 the same sub-group it differs but little from that met with in certain 

 of the tentoria sub-group. The shield is altogether absent in the 

 single specimen to which van Lidth de Jeude has given the name of 

 T. stranchi. It is possible that this may be an exceptional specimen 

 of T. geometrica. 



Another extreme is presented by ociilifera. Here the shield is 

 comparatively large and triangular, the base of the isosceles triangle 

 being a little shorter than the sides. The free end, constituted by 

 the areola, is pointed. 



Thus, as regards the nuchal shield in the geometrica-gx:o\x^, there 

 is a very decided gradational series ; it is almost absent in tentoria, 

 variable in verreauxii, and is conspicuously developed in oailifera ; 

 geometrica represents a somewhat specialized, long, narrow variation, 

 which may, perhaps, be eliminated altogether. 



g. Antebrachial Shields. 



Along the front or anterior border of the fore-arm of all the 

 members of the geojnetrica-group a series of specially enlarged imbri- 

 cating scales occur which effectually cover the front opening of the 

 shell when the animal is retracted, and thereby protect the head and 

 other vulnerable parts of the anterior region. The scales vary greatly 

 in number and character in the different types, and, for the most 

 part, grade insensibly into the smaller and more uniform scales at 

 the back of the fore-arm. Certain specimens of tentoria present what 

 may be considered as the simplest condition. Three or four well- 

 defined, longitudinal rows of scales of nearly equal size are present, 

 making four or five oblique transverse rows. The scales along the 

 two middle rows are larger and more regular in size than those of 

 the marginal rows, and all are elongated and narrow. 



The variations from the tentoria type are in the direction 

 of reduction in the number of prominent scales, and the increase in 

 size of the few remaining, culminating in a single nodular shield, 

 which is much larger and stronger than the rest. At one extreme may 

 be placed geometrica, bearing only five or six moderately enlarged 

 scales, so far apart as not to be overlapping, and at the other extreme 

 is ociilifera, with one specially enlarged and thickened scale along 

 with a few moderately large ones on the upper and outer borders. 



All transition variations are to be found between the condition 

 characteristic of tentoria and that met with in oculifera, clearly 

 indicating that the one type has been derived from the other. Even 

 in tentoria specimens are sometimes found in which the scales have 

 lost somewhat of their elongated character, and one or two of the 

 middle scales have become larger than the rest ; but 

 in verreauxii it is usual to find the number of enlarged scales already 

 reduced, and a middle one much increased in size, even becoming 

 sub-conical. This condition passes insensibly into that of oculifera 



