238 TRIONYCHOIDEA. 
to be looked upon as vestiges of the marginals of other Chelonians, 
but have evidently an independent origin, like another similar 
ossification, anterior to the nuchal, which is found in Hmyda and 
Cyclanorbis. 
Prastron.—The plastron forms, as in the Marine Turtles and the 
young of all Chelonians, an incomplete shield, a median vacuity 
being present throughout life between the lateral elements. These, 
the hyo- and hypoplastra, are joined by suture, or may even (in 
Cycloderma, Emyda, and Cyclanorbis) be fused into a single bone 
as early as birth, although distinct in the embryo * ; but their inner 
and outer borders form deep dentations, and there is no connection 
with the dorsal shield. The xiphiplastra join each other and the 
hypoplastra by interlocked digitations. The three other elements, 
viz. the epiplastra (clavicles) and the entoplastron (interclavicle), 
differ greatly from their homologues in other Chelonians, each of 
these being angular or chevron-shaped, the three forming together 
a A- or X-shaped figure ; the posterior branch of each epiplastron is 
in contact with the entoplastron, and separated by the extremity of 
the latter from the hyoplastron. Some or all of the plastral bones 
develop with age superficial rugose or sculptured plates, the so- 
called plastral callosities, which may extend considerably beyond 
the limits of, and differ greatly in shape from, the original bones. 
In Trionyx these plates are constantly present on the hyo-hypo- 
plastra, usually on the xiphiplastra, and more rarely, and as a rule 
only in fully adult specimens, on the entoplastron; in Cycloderma, 
Emyda, and Cyclanorbis, each bone has its sculptured plate, and in 
the latter genus there are a pair of additional similar plates in 
front of the epiplastra and often other, smaller, independent ossifi- 
cations as well. 
CrrvicaL VERTEBR#.—In these essentially ‘‘Cryptodiran” Turtles 
the neck is more perfectly adapted for complete and rapid retraction 
than in any other Chelonian tT, The cervical region at least equals, 
and usually exceeds, the length of the dorsal vertebral column. The 
most remarkable peculiarity resides in the structure of the last or 
eighth vertebra, the centrum of which terminates in a thin plate and 
does not articulate with the first dorsal; the articulation being 
merely by means of the zygapophyses, in such a way that the two 
vertebra form together an angle when the neck is stretched out, 
and are applied closely by their lower surfaces when the neck is 
retracted. 
The four elements of the atlas are distinct, viz. the neural arches, the 
hypapophysis (pseudocentrum), and the centrum (odontoid process). 
The following vertebree are much elongate, compressed in the middle, 
and transverse processes are absent, or merely represented on the 
* Anderson has found the hyoplastron formed of two bones, an outer and 
an inner, in an embryo of Hmyda scutata. J. Linn. Soe. xii. 1876, p. 514. 
t Anderson (Anat. Zool. Res. Yunnan, p. 785) remarks that the neck when 
retracted is so doubled on itself, that the base of the cervical vertebra, at the 
anterior extremity of the carapace, is on the same line with the tip of the snout, 
and the posterior bend opposite the inguinal notch of the plastron. 
