File:The Quarterly journal of the Geological Society of London (13859768545).jpg

182
PKOF. OWEN ON AN EXTINCT CHELONIAN
Pig, 2. Anterior Portion of the Plastron of Notochelys costata
(J nat. size).
hpt.r
In both elements of the plastron the exposed surface has lost some
substance near the mesial margin. Sufficient, however, remains to
throw welcome light on the affinities of this Chelonite.
The characters of the carapace might be interpreted as those of a
true Turtle (Chelone) not modified sufficiently to bear a subgeneric
distinction. But those of the plastron show the well-marked cha-
racters of the part in Trionyx and Chelys

the hyosternal and hy-
posternal, which are separated by a persistent transverse suture in
Chelone as in Emys and Testudo, have coalesced, and so completely in
Notochelys as to leave no trace of the original presence of the suture in
the immature reptile. The hyosternal element in Chelonians where
the plastron is best ossified, as in the extinct Eocene kinds, does not
extend backward beyond the second vertebral scute on the fourth
neural plate

whereas the single plastral bones (fig. 2, hjos),
extend backward beyond the third vertebral scute (fig. 1, v 3), and
probably beyond the fifth neural plate

for the hindmost angles of
both plastral bones have suffered fracture. Sufficient is preserved,
laterally, to show that the carapace and plastron were not united

Compare Hist, of Brit. Foss. Kept. pi. I (Chelone breviceps), pi. 13 (Chelone
longiceps), pi. 14 (Chelone convexa). See also Cuvier, Ossemens Fossiles, torn. v.
pt. ii. (4to, 1824) pi. xiii. fig. 6 (Chelone mydas), fig. 7 (Chelone