THALASSEMYDID&. 127 
The present writer unites what have usually been regarded as two distinct groups of turtles. 
For the one there have been employed the names Elvalesserivdes. Thalassemydide, Eury- 
sternida, Acichelyida, for the other the family names Propleuridze, Lytolomide, C helonemy- 
dide. The first includes mostly Jurassic genera, the second, Cretaceous and Lower Tertiary 
forms. Recently a number of writers have grouped the latter in the family Cheloniidz. 
The relationships of the Jurassic genera to Osteopygis and its kindred appear to be too 
close to permit the recognition of two distinct families. Much remains to be learned about the 
skulls of both groups; but so far as they are known, there appear to be no violent disagree- 
ments. In all, the temporal region 1s more or less completely rooft over by bone, and the jaws 
are, usually at least, fitted for crushing hard food. Nevertheless, among the Cheloniida, 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 4 plax (Eurysternum) 
and Osteopygis. Zittel’s figure of the former ( (Palaontographica, xx1v, pl. xxvii) shows that 
the hyoplastron sent fouwaad a long process that came into contact with the second peripheral, 
just as in Osteopygis. The hy poplastron, too, reacht backward to the eighth peripheral. 
There 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 Cheloniidz, the Jurassic Thalassemydide have been regarded 
as the source from which our living sea-turtles have been originally derived. 
Key To THE GENERA OF THALASSEMYDID. 
es A pit in the second peripheral for the hyoplastron. 
At least the two anterior peripher: als suturally joined with disk of carapace. Rib of 
eighth costal entering a pit in the tenth peripheral. Shell smooth or pitted Osteopygts 
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. 
c. Rib of eighth costal a i in eleventh peripheral; the skull and lower 
jaw cee n. E ... Catapleura 
bh. None of the peripherals, so far as known, articulating witb dhe disk. 
d. Lower jaw not furnisht with a beak. 
Symphysis of lower jaw not more than two-thirds the width of the 
jaws at the mental foramina F Lytoloma 
e. Symphysis three-fourths the width given . Erguelinnesta 
dd. Lower jaw with beak. Skull with rooft temporal region, Choan in 
anterior half of roof of mouth. Shell not known ; Rhetechel ys 
Genus OSTEOPYGIS Cope. 
Carapace including 8 pairs of costal plates and 11 pairs of 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 from the second to the eighth 
peripherals, not by close sutures. Bridge relatively narrow. Fontanels in the midline and at 
the ends of the hyohy poplastral suture. Inframarginal scutes present. Lower jaw with a 
broad and flat crushing surface; not beakt. 
Ty pe: Osteopygts emarginatus Cope. 
This genus was establisht by Cope in 1868 ( Proc. Acad. Nat. Sci. Phila., p. 147), and 
had for its type O. emarginatus. With it were included these species described at a later date: 
O. sopitus, Om helydrinus, and O. repandus. In his next publication on the subject (Amer. 
Naturalist, 11, 1869, p. 88) Cope subdivided the genus, setting up Prople ura with O. sopitu 
(=O. borealis) as the type. The two genera were 1 regarded as differing i in this, that Osteopygis 
had all the heal suturally ainited with the disk of the carapace, ile in Propleura nee 
the most anterior peripherals were so joined. 
