ON THE VERTEBRATE SKELETON. 253 
the two or three anterior caudal vertebre of certain flat-fishes (Pleuro- 
nectide*), characterized as usual by the simple parapophysi:l hzmal arch, 
In most air-breathing vertebrates the sacrum is characterized by both modifica- 
tions, which are carried out to their extreme in birds: in no other class is so 
large a proportion of the vertebral column converted into a ‘sacrum’ by 
coalescence (e. g. seventeen vertebrz in Struthio) : in none is the diverging 
appendage developed to such enormous proportions (e. g. Apteryx, Dinornis). 
The centrums of the middle sacral vertebre (fig. 27, ¢ 1-4) are expanded 
transversely, but depressed, and converted into horizontal plates: the neur- 
apophyses (ib. n 1-4) are lofty, expanded, and arch over the dilated part of 
the neural canal, lodging the great sacral enlargement of the myelon, with 
its ventricle. In the young ostrich, before the general anchylosis is completed, 
the bases of these neurapophyses are found to cross the interspaces of the 
centrums, and to rest equally upon two of those elements. This modifica- 
tion was retained throughout life, unobliterated by anchylosis, in the sacrum 
of the extinct dinosaurs (Jgwanodon, Megalosaurus, Hyleosaurus), and it 
obtains in the dorsal vertebra of the chelonians. The adjoining portions 
of the centrums and neurapophysis extend outwards into a short parapo- 
physis, which affords an articular surface of three facets for the short pleur- 
apophysis. One of these elements is figured iz situ at pl, fig. 27 ; it expands 
at its distal end, and coalesces there with the contiguous pleurapophyses : 
the long diapophyses (d, d) abut against the inner side, and the ilium applies 
itself to the outer side of these expanded and anchylosed ends of the short 
sacral ribs. The spinous processes of the sacral vertebrz (s, s) are developed 
antero-posteriorly, and soon coalesce into a lofty longitudinal crest of bone. 
In the chelonians, the dorsal spines develope horizontal plates from their ex- 
tremities, which unite by suture to the similarly united and expanded pleura- 
pophyses, forming with them the ‘carapace. The ‘plastron’ is formed of 
the flattened and expanded hemal spines, which are divided in the middle 
line, and have an intercalated bone (entosternal) between the halves of the 
central pieces. Professor Miiller has noticed the sacral pieurapophyses in 
the human and other mammalian embryost+. 
_ As the segments of the endo-skeleton approach the end of the tail, in the 
air-breathing vertebrates, they are usually progressively simplified ; first by 
the diminution, coalescence and final loss of the pleurapophyses ; next by the 
similar diminution and final removal of the hzmal and neural arches ; and 
sometimes also by the coalescence of the remaining central elements, either 
into a long osseous style, as in the anourous batrachia, or into a shorter 
flattened disc “which has the shape of a ploughshare},” as in many birds. 
The coalesced representative of the terminal vertebral centrums is developed 
principally from the outer layer of the fibrous eapsule of the primitive noto- 
chord. In fishes, however, the seat of the terminal degradation of the verte- 
bral column is first and chiefly in the central elements, which, in the homo- 
cercals §, are commonly blended together and shortened by absorption, whilst 
both neural and hzmal arches remain, with increased vertical extent, and 
indicate the number of the metamorphosed or obliterated centrums. 
* Hunterian Lectures on Vertebrata, 1846, p. 65, fig. 22. 
T ‘‘Selbst am Kreuzbeine mehrere Thiere giebt es noch abgesonderte Querfortsatze oder 
Rippenrudimente.”—Anatomie der Myxinoiden, heft i. 1834, p. 239. 
t “*La derniére de toutes (des vertébres de la queue), 4 laquelle les pennes sont attachées, 
est plus grande et a la forme d’un soc de charrue, ou d’un disque comprimé :—dans le jeune 
age, elle est évidemment composée de plusieurs vertébres.”—Cuvier, Lecons d’Anat. Comp. 
2d ed. i. p. 208, and “ Lawrence’s Blumenbach’s Comparative Anatomy,” ed. 1827, p. 62. 
§ M. Agassiz’ expressive name for the fish with a symmetrical bilobed tail. 
