THAI.ASSKMYmn.'h. 



127 



I Ik present writer unites what have usually been regarded as two distinct groups or turtles. 

 For the one there have been employed the names Thalassemydes, Thalassemydidae, Eury- 

 sternidae, Acichelyidae, tor the other the family names Propleuridae, Lytolomidas, Chelonemy- 

 dida:. The first includes mostly Jurassic genera, the second, Cretaceous and Lower Tertiary 

 tonus. Recently a number of writers have grouped the latter in the family Cheloniidae. 



The relationships of the Jurassic genera to Osteopygis and its kindred appear to be too 

 close to permit the recognition ot two distinct families. Much remains to be learned about the 

 skulls ot both groups; but so far as they are known, there appear to be no violent disagree- 

 ments. In all, the temporal region is more or less completely roott over by bone, and the jaws 

 are, usually at least, fitted for crushing hard food. Nevertheless, among the Cheloniidae, and 

 some other families, there is the greatest variation in respect to the character of the jaws. 



A remarkable resemblance is to be observed between the shells of A pla\ (Eurysternum) 

 and Osteopygis. Zittel's figure of the former (Palaeontographica, xxiv, pi. xxvii) shows that 

 the hyoplastron sent forward a long process that came into contact with the second peripheral, 

 just as in Osteopygis. The hypoplastron, too, reacht backward to the eighth peripheral. 

 I here were, likewise, extensive fontanels enclosed by the outer ends of these plastral bones 

 and the peripherals. 



As indicating on the part of writers a recognition of close relationship between the various 

 elements united here into one family, it may be recalled that, while the later forms have been 

 actually incorporated with the Cheloniidae, the Jurassic Thalassemydidae have been regarded 

 as the source from which our living sea-turtles have been originally derived. 



Key to the Genera of Thalassemydida. 



A . A pit in tin- second peripheral for the hyoplastron. 



a. At least the two anterior peripherals suturally joined with disk of carapace. Rib of 



eighth costal entering a pit in the tenth peripheral. Shell smooth or pitted Osteopygis 



AA. No pit, so far as known, in the second peripheral for hyoplastron. 



b. Two anterior peripherals of each side articulating with the disk of the carapace. 

 e. Rib of eighth costal entering pit in eleventh peripheral ; the skull and lower 



jaw unknown Cataplem n 



bb. None of the peripherals, so far as known, articulating with the disk. 

 J. Lower jaw not furnisht with a beak. 



1 '. Symphysis of lower jaw not more than two-thirds the width of tin 



jaws at the mental foramina Lytoloma 



,,. Symphysis three-fourths the width given . . Erquelinnesia 



Jil. Lower ]aw with beak. Skull with rooft temporal region. Choanae in 



anterior half of roof of mouth. Shell not known Rhetechelys 



Genus OSTEOPYGIS Cope. 



Carapace including 8 pairs of costal plates and I I pairs ot peripherals, with all or only a 

 part of the peripherals suturally articulated with the costals and the suprapygals. All the 

 costals sending the ends of their ribs into pits of the corresponding peripherals. Five vertebral 

 and 4 pairs of costal scutes. The nuchal scute much wider than long. Plastron relatively 

 small; its connection with the peripherals extensive, reaching trom the second to the eighth 

 peripherals, not by close sutures. Bridge relatively narrow. Fontanels in the midline and at 

 the ends of the hyohypoplastral suture. Inframarginal scutes present. Lower jaw with a 

 broad and flat crushing surface; not beakt. 

 Type; Osteopygis emarginatus Cope. 



This genus was establisht by Cope in 1808 (Proc. Acad. Nat. Sci. Phila., p. 147), and 

 had for irs type 0. emarginatus. With it were included these species described at a later date: 

 0. sopitus, 0. chelydrinus, and 0. repandus. In bis next publication on the subject (Amer. 

 Naturalist, m, i86y, p . 88) Cope subdivided the genus, setting up Propleura with 0. sopitus 

 (). hrjn-nlis 1 as the type. The two genera were regarded as differing in this, that Osteopygis 

 had all the peripherals suturally united with the disk of the carapace, while in Propleura only 

 the most anterior peripherals were so joined. 



