Nov. 2 1, 1895. SKELETON OF Protostega gigas HAY. 59 



Had the breadth of the body of Protostega possessed the same 

 ratio to the length that we find existing in Thalassochelys, the lower side 

 of the animal would have been about 2.2 metres wide. The positions 

 of the surfaces for union with the epiplastra and xiphiplastra, and the 

 location of the axis of strongest development of the two plastral bones 

 of each side make it evident that the outer border of the bony plas- 

 tron was at a considerable distance from the outer edge of the body. 

 This is shown too by measuring outward from the excavation for the 

 arm a distance proportional to that found in Thalassochelys. The tips 

 of the digitations of the plastral bones must have lacked as much as 

 30 cm. of reaching the marginal bones. This will leave a space of 

 about 120 cm. from the bottom of the excavation for one arm to that for 

 the other. When we come to compare the distance from the hinder to 

 the front excavations, in the restoration of Protosphargis by Capellini 

 (Mem. Ac. dei Lincei, 1884, 3 ser., vol. 18), with the distance of 

 the two anterior excavations apart, I find that the two measure- 

 ments have almost exactly the same ratio that I have given them in 

 Protostega. 



If we have placed the plastral bones aright, there is left between 

 them a great fontanelle. Where the hyoplastra are widest this is 

 about 43 cm. in width ; and opposite the union of the hyo- and hypo- 

 plastron, about 90 cm. This is somewhat smaller, however, than the 

 fontanelle found in Protosphargis, and much smaller than that of 

 Dermochelys. 



The nearest relative of Protostega is undoubtedly Protosphargis \ 

 but when we come to compare the two plastra, we find abundant dif- 

 ferences. That of Protosphargis is considerably less developed than 

 that of Protostega. Notwithstanding this, there was on the front of 

 the hyoplastron of Protosphargis a long slender process which ran for- 

 ward and inward to connect with the epiplastron. In Protostega the 

 corresponding angle of the hyoplastron is broad, rounded off, and 

 digitated. In Protosphargis again there is a broad notch in the ante- 

 rior and outer border of the hypoplastron, but none in Protostega. 



It appears to be quite evident that Capellini's restoration of Pro- 

 tosphargis is in one respect not wholly accurate. The epiplastra ap- 

 pear to be too short and to converge too rapidly, thus making the 

 plastron too short. 



Accompanying the plastral bones here described is another bone 

 which must he regarded as the nuchal. Considerable portions of it 

 are wanting at each lateral extremity ; and the tip of the process which 

 projects backward toward the first dorsal neural arch is also broken 

 away. The portion of the bone remaining projects outward on each 



