198 EEPTILIA class m 



characters regarded as ancestral to both. Plastron with larger bones and a 

 much smaller fontanelle than in Dermochelys. Body elongate, the posterior 

 end truncated ; limbs paddle-shaped. Post-axial border of humerus more or 

 less deeply emarginate. Upper Cretaceous ; North America. 



Archeion, Wieland. Larger than the preceding, but similar in most 

 respects. Ft. Pierre Cretaceous ; South Dakota. 



Protosphargis,^ Cap. Skull unknown. Body skeleton similar to that of 

 Protostega, but bones of plastron more slender, and median fontanelle larger. 

 Uppermost Cretaceous ; Italy. 



Pseudosphargis, Dames. Skull flat and wide, like that of the Dermo- 

 chelyidae, but with descending parietal processes. Oligocene ; Germany. 



Family 3. Ohelonidae. Gray. (Marine Turtles.) 



Shell more or less incompletely ossified, covered loith epidermal shields. Carapace 

 frequenfly cordiform. ; plastral bones nine, distinä from the carapace, and with vacuities 

 and digitale lateral extremities ; nuchal without costiform process. Temporal region 

 of skull completely roofed over ; squamosal joining the parietal, and the latter articn- 

 lating with the postfrontal. Limbs paddle-shaped, phalanges without condyles, terminal 

 claws reduced to one or two. Supramarginal shields present, and sometimes an inter- 

 gular. Upper Cretaceous to Recent. 



Osteopygis, Cope (Catapleura, Cope). Carapace practically closed; marginals 

 eleven. Upper Cretaceous. 



Allopleuron, Baur. Carapace long and narrow, nuchal deeply emarginate, 

 neurals short and wide with a long keel ; marginals long and slender. Upper 

 Cretaceous. 



Lytoloma, Cope {Euclastes, Puppigerus, p.p., Cope ; Glossochelys, Seeley ; 

 Erquelinnesia, Pachyrhynchus, Dollo). Skull resembling that of Thalassochelys. 

 Carapace rounded posteriorly, vacuities of shell more obliterated than in 

 Thalassochelys ; epiplastrals narrow ; exposed portion of entoplastron very 

 short , xiphiplastrals uniting extensively in the median line. Upper Creta- 

 ceous and Eocene. 



Argillochelys, Lyd. Skull short and wide. Shell and bones of the pectoral 

 arch similar to those of Thalassochelys, except that the carapace has but four 

 costal shields, and the xiphiplastrals unite extensively along the median line. 

 Eocene ; England. 



Thalassochelys, Fitz. (Fig. 301). Carapace completely ossified in the adult, 

 with at least five pairs of costal shields, and vacuities more or less obliterated. 

 A series of inframarginal plastral shields present. Eocene to Eecent. 



Chelone, Brong. (Mydas, Gray ; Cimochelys, Owen), (Figs. 297, 302). Skull 

 comparatively long, narrow, with the orbits lateral. Shell cordiform or 

 pointed at both ends, with four costal shields, the plastral vacuities persisting 

 for a long period. An intergular and a series of inframarginal shields present. 

 Upper Cretaceous to Recent. 



Lembonax, Cope. Eocene ; North America. Chelyopsis, van Beneden. 

 Oligocene ; Belgium and Northern Germany. 



During the Cretaceous and Tertiary numerous marine turtles existed which 

 are perhaps most nearly related to the Ohelonidae, but owing to their diversity 

 have been placed by some authors in independent families. Most of them. 



