CRYPTODIRA. 15 



in the Chc'lydridic and Cinostcrnida;, narrowing iu 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 liypojdastra with the xiphiplastra. It is to be noted that 

 the axilhiry ai:d ingniual buttresses, when present, ossify very early, 

 and are nearly as much developed in the young as in the adult. 



Cervical Yektebr.e. — 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, Pi/.vis, in which they are all ])roca;lous in the 

 specimen examined by Yaillant, as well as in the one in the iSritish 

 Museum, hut the mode of articulation varies considerably in 

 certain of the median vertebrtc, 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 Testudinidic and Platy- 

 sternida' ; the last four are procoelous in the Chelydridtc, Dermate- 

 mydida^ Cinosternida\ and Chelonidic. 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 I'leurodira. 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, llypapo- 

 physes may be present between some of the vertebrae, single between 

 the first and second, in pairs between the others (Emijs, Chmmi/s, 

 CJivlonc). In old age those ossifications may coalesce with the 

 posterior inferior border of the centra f. 



Sacral and Catjdal Vertebr^tj.- — The sacral vertebrae are two in 

 number, their centra are bi])lane. The ribs are, as on the last 

 dorsal vertebra^ 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 Chelydrida3 and 

 Chelonidii?, weakest in the terrestrial Testudinida^. 



The caudal vertebra), as far as my experience goes, range in 

 number from 16 {Cistudo) to 35 (CJielydra, Emys orhicidaris) ; the 

 more usual number being from 20 to 25. The comparative length 

 of the tail is not indicative of the number of vertebra) ; it is also 

 remarkable that the two extreme numbers occur in two closely 



* C. K. Hofimann regards these " paradiapopLyses" as rudimentary ribs ; in 

 embryos there is an autogenous ossification between the neural arch and the 

 centrum. In the adult Chelonidje, in which the neuro-ccnti-al suture persists 

 ■very distinct thruughcut life, the rib is represented by a cartilaginous tubercle. 



t For fuller details on the cervical vertebra?, cf. Yaillant's excellent memoir 

 in Ann. So. Nat. x. 1880. 



