930 THE AMERICAN NATURALIST. (VoL. XXXII. 
too great, his own calculation making the total length 2.273 
meters, as against my own estimate of 3.92 meters—a very 
considerable difference. : 
However, I do not believe that Case’s conclusion necessarily 
follows from his premises. My estimate was primarily based 
on the distance from the bottom of the excavation in the hypo- 
plastron for the fore limb to the excavation in the hypoplastron 
for the hind limb. It seems to me highly probable that these 
borders of the plastron could not have approached the corre- 
sponding limbs of Protostega more closely than they do in 
Thalassochelys and yet leave the limbs free to make their 
movements. The limbs, then, must have been as far apart as 
they would be in a Thalassochelys whose plastron had the cor- 
responding measure equal. Hence any shortening of the body 
must have been effected in front of the fore limb and behind 
the posterior limbs. This would necessitate the shortening 
of the anterior dorsal and the anterior caudal vertebrz ; and of 
this we have no proof. The dimensions of the various plastral 
elements are extremely variable in the various genera of turtles ; 
and it hardly follows that, because the xiphiplastrals are very 
short, the body of the animal is correspondingly curtailed. 
The estimate of the length made by Wieland, based on his 
apparently quite perfect carapace, is not greatly less than my 
own estimate. 
I wish here to make a remark on the genus Atlantochelys of 
Agassiz. It has been thought that it is identical with Protos- 
tega of Cope; but a comparison of Leidy’s figure! of the 
humerus, on which the name was based with that of Protostega, 
shows that the two genera are very distinct. The humerus 
of Atlantochelys contracts below the tuberosities into a much 
more slender shaft than does Protostega. The humerus of At 
lantochelys mortoni resembles not distantly that of Lytoloma, as 
figured by Dollo.? 
It was the judgment of Baur,® also, that Atlantochelys is 
different from Protostega. Cope’s P. neptunia is merely a 
1 Cretaceous Reptiles, U. S., Pl. VIII, Figs. 3-5. 
266. 
2 Geol. Mag. [3], vol. v, p. 2 
3 Biolog. Centralblatt, Bd. ix, p. 189. 
