10 



Annals of the Carnegie Museum. 



by predaceous fishes and sharks, or by wave action, after the turtle 

 went down, and thus explain the rarity of its recovery and why the 

 explorations of thirty years have hitherto failed to reveal so interesting 

 a skeletal part. 



Inasmuch as the adequate mounting and preparation for description 

 of the original type of Archelon ischyros has now been begun at the 



Yale Museum, there is no pres- 

 ent need to attempt further 

 plastral restoration of this largest 

 of sea turtles. Meanwhile, how- 

 ever, a very good idea of the 

 plastral form may be had by 

 comparison, in combination, of 

 the writer's figure of the ento-, 

 hyo-, hypo-, and xiphiplastra 

 of Archelon with the figure of 

 the Jurassic Thalassemyd Hydro- 

 pelta Meyeri given on page 530 

 of Vol. III. of Zittel's Hand- 

 buch. The manner in which 

 the epiplastra of Archelon pro- 

 jected anteriorly is quite closely 

 paralleled in Hydropelta, except 

 that in the former it appears that 

 there was no epi plastral abutting 

 on the median line, and that the 

 entoplastron is relatively larger. 

 The present determination for the first time of the true type of 

 plastron in the Protosteginre is of far more than casual interest be- 

 cause of the obvious bearing on the most vexed of zoopaleontological 

 problems, the origin of Dermochelys, as well as on the highly inter- 

 esting question of the mono- and polyphyly of the other existing and 

 the various extinct genera of marine turtles. 



The testudinate plastron while undergoing characteristic variations 

 of form within closed groups is fully as conservative a structure as the 

 carapace. Also, since in Dermochelys the plastron and nuchal are the 

 only parts left for comparison with the normally developed carapace 

 and plastron of other Testudinates, the paleontologic record has been 

 scanned year after year for true marine turtles with a more or less reduced 



Fig. 3. DermocJielys coriacea. Plastron 

 with (a) ectal and (b) ental view of nuchal. 

 After Gervais {cf. out-turned epiplastra 

 with those of Archelon, etc., in subjoined 

 figures ) . 



