36 MR. ST. GEORGE MIVART ON THE 
VERTEBRA OF APTERYX (twice the natural size). 
Fig. 33. Fig. 34. 

Fig. 33. Dorsal view of third vertebra. Fig. 34. Dorsal view of fourth vertebra. 
Fig. 35. Dorsal view of fifth vertebra. Fig. 36. Dorsal view of sixth vertebra. 
ac, preaxial surface of centrum; az, prezygapophyses; d, the diapophysial element ; kp, the hyperapophysial element ; 
ns, neural spine ; ps, postaxial surface of centrum; pz, postzygapophyses, 
ing the diapophysis with the parapophysis develops a tubercular process on its outer 
surface, which projects postaxiad and increases the complexity of the skeleton of the 
neck (fig. 37). The process is in series with that part of each thoracic rib which pro- 
ceeds distad from the tuberculum. 
In the tenth and eleventh vertebre (as in dA. owenii, but not in A. australis) the cata- 
pophyses, which have been increasing in conspicuousness and mesiad flexion, from the 
fourth vertebra, unite together and form a subyertebral ring. 
In the twelfth vertebra a median hypapophysial process begins to appear, and the 
hyperapophyses become more closely approximated to the posterior zygapophyses. 
These characters are more marked in the thirteenth vertebra, in which vertebra, in A. 
owenii, the hypapophysis attains its maximum of development, though in A. australis 
it is largest in the vertebrae from the fourteenth to the seventeenth inclusive. 
In the fourteenth vertebra the hyperapophyses and posterior zygapophyses have 
become completely blended together, so that the former cease to be distinguishable ; 
and this vertebra is moreover distinguished from all the cervical vertebree which are 
