ON THE VERTEBRATE SKELETON. 263 



the two or three anterior caudal vertebrae of certain flat-fishes (Pleuro- 

 nectidce*), characterized as usual by the simple parapophysial haemal 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 vertebras in Struthio) : in none is the diverging 

 appendage developed to such enormous proportions (e. g. Apleryx, Dinornis). 

 The centrums of the middle sacral vertebrae (fig. 27, c 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 (Iguanodon, Megalosaurus, Hylceosaurus), 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 in situ at pi, 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 vertebrae (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 haemal 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 pleurapophyses in 

 the human and other mammalian embryos f- 



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 haemal 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 J," as in many birds. 

 The coalesced representative of the terminal vertebral centrums is developed 

 principally from the outer layer of the fibrous capsule 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- 

 cereals §, are commonly blended together and shortened by absorption, whilst 

 both neural and haemal 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 melirere Thiere giebt es noch abgesonderte Querfortsatze oder 

 Rippenrudimente." — Anatomie der Myxinoiden, heft i. 1834, p. 239. 



+ "La derniere de toutes (des vertebres de la queue), a laquelleles pennes sont attachees, 

 est plus grande et a la forme d'un soc de cbarrue, ou d'un disque comprime : — dans le jeune 

 age, elle est evidemment composee de plusieurs vertebres." — 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. 



