12 CRYPTODIRA. 
axillary buttress of the plastron ; this first rib is considerably more 
elongate in the Chelydride. From the second to the eighth ver- 
tebra inclusive the centra, which have flattened articulary faces, 
are connected by suture with the neural arches, which expand into 
bony (neural) plates alternating with the centra, and with strong 
ribs which, at a short distance from their attachment (between 
centrum and arch), expand into large bony (costal) plates suturally 
united with the neural plates. ‘The neural spine may be low or 
obsolete, or more elevated, in which case it is reduced to a narrow 
septum pierced with fontanelles; the latter being special to adult 
specimens of the genus Testudo. The “ capitular” extremity of the 
second rib articulates between the first and second centra, and so do, 
as a rule, the third and fourth; but the following generally shift 
back so as to articulate with the middle of the posterior centra. In 
very old specimens it not unfrequently happens that the costal 
“capitula” become atrophied or disappear altogether. The tenth 
vertebra forms the counterpart of the first: the centrum is very 
short, with a posterior condyle, and a short rib which may (in 
Chelydra and Platysternum) abut on the eighth rib in the same 
manner as the first does on the second; but, as a rule, it differs 
little in appearance from the penultimate dorsal and the following 
sacral. With the exception of the first and hindermost, the centra 
are longer than broad: they may be flattened inferiorly (second 
vertebra of most Emydines, second to fourth or fifth of Chelydridz), 
feebly compressed, or strongly compressed and forming a keel (Der- 
matemys, Batagur, Chrysemys, Testudo). 
In front of the series of neural plates is a large plate, the nuchal, 
situated above the first dorsal vertebra: this plate, which is present 
in all Chelonians, is a cartilage bone, developed simultaneously with 
the neural plates; the fact that it 1s connected, by ligament, with 
the last cervical vertebra, and that it assumes on each side, in the 
Chelydridie, the form of a rib, suggests the possibility of its repre- 
senting, in part at least, the modified ribs of the last cervical. Such 
rib-like processes are also present, well developed in the young and 
shorter in the adult, in the Dermatemydidze and Cinosternide, 
underlying the first or first two marginals ; in Chelydra and Ma- 
croclemmys the rib-like process extends to below the third marginal 
and abuts on the distal end of the first costal. A mere indication 
of this process is found in some young specimens of Hmys orbicularis 
and other Emydines. The nuchal is six-sided and broader than 
long *, 
There are normally eight neural plates. In a few genera, how- 
ever (Dermatemys, Staurotypus, Cinosternum, Cistudo), the series is 
reduced by one to three of the hindermost, and the corresponding 
last or last two or three pairs of costal plates meet in a suture on 
* A fossil Chelydra (Chelydropsis, C. Peters) has been described as having 
two nuchals; but that such is due to an anomalous transverse division, I have 
little doubt. 
