CRYPTODIRA. 1S} 
in the Chelydride and Cinosternide, narrowing in most other 
genera, in which each of these bones forms three diverging branches. 
The hyoplastra are constantly in contact with the epiplastra, and so 
are the hypoplastra with the xiphiplastra. It is to be noted that 
the axillary and inguinal buttresses, when present, ossify very early, 
and are nearly as much developed in the young as in the adult. 
CrrvicaL VERTEBR&.—The cervical vertebra, which number eight 
as in all Chelonia, present this peculiarity, that their centra exhibit 
the four modes of articulation, some being concavo-convex, others 
convexo-concave, others biconvex, others biconcave. A single ex- 
ception is known, Pyvis, in which they are all proccelous in the 
specimen examined by Vaillant, as well as in the one in the British 
Museum. But the mode of articulation varies considerably in 
certain of the median vertebrae, not only according to species, but 
even sometimes according to individuals. With the exception 
noticed above, the first vertebra is biconcave; the last is biconvex, 
and the penultimate biconcave in the Testudinidee and Platy- 
sternide ; the last four are proccelous in the Chelydride, Dermate- 
mydidw, Cinosternide, and Chelonide. The articulation between 
two or more of the posterior centra is by means of two condyles 
fitting into two sockets, and this ginglymoid articulation, which is 
opposed to any lateral movements, is diagnostic of the Cryptodira 
as contrasted with the Pleurodira. Transverse processes are absent 
or merely indicated *. The centrum of the first vertebra (odontoid 
bone) is constantly distinct from the three elements (hypapophysis 
and neurapophyses) which articulate with the skull. Hypapo- 
physes may be present between some of the vertebree, single between 
the first and second, in pairs between the others (Himys, Clenmys, 
Chelone). In old age these ossifications may coalesce with the 
posterior inferior border of the centra 7. 
SacraL AND Caupat VerrEBRa.—The sacral vertebre are two in 
number, their centra are biplane. The ribs are, as on the last 
dorsal vertebrae, suturally united with both centrum and neural 
arch ; the first is the most developed, considerably expanded dis- 
tally. The sacral ribs are strongest in the Chelydride and 
Chelonidx, weakest in the terrestrial Testudinide. 
The caudal vertebrae, as far as my experience goes, range in 
number from 16 (Cistudo) to 85 (Chelydra, Emys orbicularis); the 
more usual number being from 20 to 25. The comparative length 
of the tail is not indicative of the number of vertebre ; it is also 
remarkable that the two extreme numbers occur in two closely 
* C. K. Hoffmann regards these “ paradiapophyses” as rudimentary ribs; in 
embryos there is an autogenous ossification between the neural arch and the 
centrum. In the adult Chelonide, in which the neuro-central suture persists 
very distinct throughout life, the rib is represented by a cartilaginous tubercle. 
t+ For fuller details on the cervical vertebrae, ¢f. Vaillant’s excellent memoir 
in Ann. Se, Nat. x. 1880. 
