PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 439 



plates are twenty-four iu number. The nuchalis irregularly quadrilateral, 

 broadest behind. The supra-caudal is single, large, twice as long as high, 

 and but slightly bulging. It stands in an almost vertical position. The 

 first marginal plate is irregularly pentagonal; the second and third quad- 

 rilateral or pentagonal; the fourth ijentagonal; the fifth quadrilateral; 

 the sixth pentagonal; the seventh to the eleventh, inclusive, quad- 

 rilateral. All the plates are about equally striated with concentric 

 lines. 



The sternum is very thick, and in adults extends about an inch be- 

 yond the anterior edge of the carapace. The gular j)lates together 

 form an eloogated pentagon, but there is sometimes a deep notch 

 between them anteriorly. The brachial plates are quadrilateral, the 

 free border longest; the anterior borders, which receive the posterior 

 sides of the gular plates, together cresceutic in outline. The surface of 

 these plates, unlike that of those of X. polypJiemus, is level in the an- 

 tero-posterior direction. The thoracic plates are rudelj^ quadrilateral, 

 narrow, and but little expanded at their outer extremities. The abdom- 

 inal plates are nearly quadrilateral, but less nearly square than in X. 

 polyphemtis. The femoral plates are also rudely quadrilateral, but much 

 narrower than in X. polypliemus^ the interior lateral border being 

 scarcely more than half the length of the anterior side. The subcaudal 

 plates, which do not differ from those of the eastern species, are rudely 

 quadrilateral, the interior lateral border being a little shorter than the 

 exterior. 



These characters, as iu all tortoises, are quite variable and unsatis- 

 factory. 



Fig. I.— X. POLYPHEMUS. Fig. II.— X. Agassizii. 



Contour of the head shown by passing a plane through the lowest point of the 

 orbit parallel to the upper surface of the head. 



The head is considerably compressed at the sides and elongated. Its 

 superior surface is covered with flat scales, which decrease in size back- 

 ward, and are usually divided into pairs between the eyes, and very large. 

 The nostrils are quite small and near together, and are raised a little above 

 the upper edge of the sheath of the upper jaw. The eyes are large and 



