PEOFESSOR OWEN ON THE GENUS DINORNIS. 



175 



('dorsal' or upper) view (fig. 61, loc. cit.). 

 The modilications of sacral structure here ex- 

 hibited, which have proved most instructive 

 in their application to the vertebras 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 intei-vals of the centrums is the rule 

 in dorsal vertebrae, and a like disposition of 

 the neural arches occurs in the dorsal vertebrae 

 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 Iguanodon and other large extinct 

 Rejitilia 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" i. 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 vertebrae 

 yielded the key to the characters of the 



Pig. 35. 



D. crassus. 



D. elephantopus. 



D. robustus. 



D. maximus. 



D. giganteits. 



D. rhe'ides. 



Specific modifications of the ster- 

 num in the genus Dinornis. 



individual vertebrae to be completed" (Zool. Trans, viii. p. 420). Probably my preparation, No. 1885, may be 

 here alluded to. 



' ' Eeports of the British Association,' 8vo, 1841, " On Brilish Fossil Reptiles," p. 106. = lb. ib. 



