58 FIELD COLUMBIAN MUSEUM ZOOLOGY, VOL. i. 



The length of the hyoplastron is 6icm; its width 52.5 cm. The 

 extreme width of the hypoplastron is somewhat less than that of the 

 hyoplastron, but it cannot be accurately determined. The latter 

 bone is thickest just behind and somewhat mesiad of the excavation 

 for the fore limb, and here the thickness amounts to 45mm. The 

 hypoplastron is not so thick, but still quite thick and solid. The cor- 

 responding bones in Professor Cope's possession were not more than 

 half an inch in thickness, at the most. This condition was in all 

 probability due to the pressure to which they had been subjected. 



As will be observed, the anterior inner angle of the hyoplastron 

 is extensively developed, surpassing in this respect that of Thalasso- 

 chelys, in which again the plastron is more developed than in 

 Chelonia. As usual in all the recent marine turtles, this angle ex- 

 tends further forward than does the outer one. To that border of 

 this angle which lies next to the fore limb was attached the hinder 

 end of the epiplastron. Neither of the epiplastra was secured. In 

 Thalassochelys the anterior ends of the epiplastra extend in front of a 

 line joining the bottoms of the excavations for the fore limbs a dis- 

 tance equal to that from the bottom of the excavations for the fore 

 limbs to those for the hind limbs. This, in the Protostega plastron 

 before me, amounts to 84 cm. The xiphiplastra of Thalassochelys 

 extend behind the excavations for the hinder limbs as far as do the 

 epiplastra from the anterior excavations. If these proportions hold 

 good for Protostega, the whole length of the plastron would amount 

 to at least 2.4 metres. 



As shown by the figure, the hinder end of the hypoplastron is 

 prolonged backward and somewhat inward as a long process. Mesiad 

 of this process there has been another and, judging from the example 

 of Chelonia and Thalassochelys, a longer process. A portion of this 

 process is missing, but the bone, where the fracture has occurred is 

 still 21 mm. thick. This missing process was also evidently directed 

 somewhat toward the middle line of the body, as well as backward. 

 Between the two processes has been received the forked end of the 

 xiphiplastron of that side. The upper end of the inner border of the 

 outer process has been chamfered off where it forms a suture with 

 the xiphiplastron. This chamfering of the bone continues beyond 

 the point of union of the two processes and is then carried backward 

 on the inner process as far as this remains. The upper side of the 

 outer border of the outer process has also entered into sutural union 

 with the xiphiplastron. The whole structure is here extremely simi- 

 lar to that seen in Chelonia and Thalassochelys. 



