288 THE GALAPAGOS TORTOISES. 



apparent, shallow, but distinct. The second, third, and the fourth marginals, 

 toward their outward ends, are slightly curved upward; the first pair above the 

 neck incline downward slightly; the hindmost one, the caudal, extends down 

 abruptly and curves forward at the lower edge; and on the fourth to the eighth 

 marginals, along the flanks, there is a low blunt keel. 



The width of the plastron is about nine elevenths of its length; the con- 

 cavities on its lower surface are so shallow at this age they may be overlooked. 

 The gular extremity is subtruncate, rounded, and a trifle swollen upward at the 

 end in front. There is a weak notch between the gulars; they turn downward 

 slightly at the front edge. The lateral outline of each humeral scale is convex 

 in its hinder half and a very little concave anteriorly, forming a feeble sigmoid; 

 and in the femoral section the lateral outline of each femoral scale is strongly 

 convex. The anal scutes are separated by a deep notch, and each scale is 

 longer than wide. In the young as in the old there is a considerable amount 

 of individual variation, in outlines, prominence of areolar spaces, smoothness, 

 and in measurements. The specimens in Plates 24, 25, 26 are a little larger 

 than the type to which the name T. nigra was originally applied; they are 

 part of the lot secured by the Hassler Expedition on Santa Maria Island 

 (Charles) . 



Plates 32 and 33 represent a group of medium sized specimens, about 

 twice the length of the type. They exhibit considerable changes worthy of 

 note in a specific description. There has been an increase in direct length as 

 compared with the direct width and height, and an increase in the width across 

 the curvature beyond that of the curved length as the width and bulging round- 

 ness of the body increases, posteriorly there is a comparative increase in the 

 height near the middle of the fourth vertebral scale and, in males especially a 

 widening or flattening of the arches of the back without corresponding increase 

 in height. The nuchal height has become greater, the humeral and the femoral 

 arches have become higher and wider proportionally, and some of the marginal 

 plates are curved and thicker at the free outer edges. The caudal plate has 

 acquired a greater prominence as the marginals at its sides have worn away. 

 The notches on the sides at the fourth and the eighth marginals, near the angles 

 of the bridge, and the grooves above them are deepened; compression of the 

 flanks anteriorly has steepened the sides and the saddle-shape, so called, has 

 been increased thereby. From the scales the striae have disappeared in great 

 measure, as also has the low ridges from the fourth to eighth marginals. 



In both sexes the plastron has become deeply concave; its ends are wider 



