60 FIELD COLUMBIAN MUSEUM ZOOLOGY, VOL. t. 



side of the middle line less than 18 cm. If the length of the bone had 

 the same ratio to the remainder of the carapace of Protostega that we 

 find in Chelonia, it should extend laterally about 40 cm. That it had 

 this length so as to reach the first marginal, is quite probable. If the 

 anterio-posterior extent of the bone were equal to that of Chclonia, it 

 would be about 30 cm. at the narrowest part; but it is only 6 cm. In- 

 deed, the portion remaining appears to represent little more than the 

 median, backwardly projecting process and the anterior thickened 

 border of the nuchal of Chelonia. The reduction in the anterio-pos- 

 terior direction really appears to have gone further than in Dermochc- 

 lys even. In the latter, however, the anterior border of the bone has 

 been removed, so that it, as well as the other borders, are jagged 

 and thin. In Protostega it is the hinder border of the bone which has 

 been removed. 



The anterior border of the bone is relatively thick, 3 cm., and is 

 somewhat bevelled, so as to look downward and forward. On the 

 upper surface, near the anterior border on each side, is a broad shal- 

 low groove. The process which is seen to extend backward from the 

 body of the nuchal probably reached the first neural. It must then 

 have had a length of about 28 cm. 



As in the case of other Cretaceous marine turtles, there is found 

 on the under surface of the nuchal no tubercle for articulation with 

 the last cervical vertebra. 



As regards the presence of a dermal carapace of mosaic-like 

 plates, such -as is found in Dermochelys, the remains here described 

 afford no light. No evidence of its presence has been furnished by 

 any of the specimens of Protostega so far produced. It is neverthe- 

 less too early to assure ourselve that there was no such a structure, 

 considering how easily it could become detached from a carcass 

 which was being tumbled about by the waves and dragged by carni- 

 vorous lizards. 



Professor Cope has described some of the vertebrae and ribs of 

 Protostega. The vertebras, like the remainder of the skeleton, had 

 been greatly flattened by pressure, and probably to this circumstance 

 is to be attributed their relatively great width. The true relation- 

 ships to the vertebral axis were thus rendered obscure. Notwith- 

 standing the possession of ball and socket articulatory surfaces, it was 

 thought that some of these vertebras belonged to the dorsal region. 

 Others were regarded as appertaining to the neck. 



The length of the shortest cervical vertebra, the first behind the 

 axis, in a specimen of Chelonia with carapace 790 mm. long is 35 mm. 

 Professor Cope's specimen of Protostega had apparently close to three 



