419 
(‘dorsal’ or upper) view (fig. 61, doc. cit.). 
The modifications of sacral structure here ex- 
hibited, which have proved most instructive 
in their application to the vertebre of extinct 
animals, are the alternate disposition of certain 
centrums and neural arches and of a few other 
centrums and pleurapophyses. In Mammalia 
such disposition of the heads of ribs across the 
articular intervals of the centrums is the rule 
in dorsal vertebrae, and a like disposition of 
the neural arches occurs in the dorsal vertebre 
of Chelonia; but the concurrence of the alter- 
nating positions of centrums with both ele- 
ments appears not to have been observed in 
the sacral region of any vertebrate until the 
task of determining the singular detached cen- 
trums in the Jgwanodon and other large extinct 
Reptilia led me to a series of researches into 
the sacral structures and their development in 
existing Vertebrates. These researches led, 
among other results, to the detection, in the 
long sacrum of birds, of ‘a shifting of the 
neural arch from the middle of the body to 
the interspace of two adjoining centrums, each 
neural arch being there supported by two con- 
tiguous vertebrae, the interspace of which is 
opposite the middle of the base of the arch 
above, and the nervous foramen is opposite the 
middle of each centrum” !. By this modifica- 
tion, “that part of the spine subject to 
greatest pressure is more securely locked to- 
gether;” and I further remarked that, “ this 
structure is beautifully exemplified in the 
sacrum of the young Ostrich” ?. 
The detached centrums of such vertebra 
yielded the key to the characters of the 
- 
i 
Dz 
\ 
Specific modifications of the ster- 
num in the genus Dinornis. 

. Crassus. 
Pee 
pt\ 
elephantopus, 
. robustus. 
. maximus. 
giganteus. 
. rheides. 
individual vertebra to be completed ” (Zool. Trans. viii. p. 420), Probably my preparation, No. 1885, may be 
here alluded to. 
 « Reports of the British Association,’ 8yo, 1841, “On British Fossil Reptiles,” p. 106. 
* Tb. ib. 
3.7 2 
Seer 
