427 



a new or accessory articular surface on the outside of the posterior zygapophysis of the ante- 

 cedent vertebra. This tenon-and-mortice articulation of the metapophysis with the zygapo- 

 physis on the inner side and with the anapophysis on the outer side, is repeated throughout 

 the whole lumbar series. The anapophysis begins to be developed from the anterior dorsal 

 vertebra, and even there presents an articular surface at its under part to join a corresponding 

 surface on a parapophysis developed from the fore and outer part of the neural arch of the 

 succeeding vertebra. In the tenth dorsal a second articular surface is established in the upper 

 part of the anapophyses for the inferior metapophysial one of the succeeding vertebra ; here, 

 therefore, the anapophysis begins to be morticed between the parapophysial and metapo- 

 physial articular surfaces, which surfaces continue to the antepenultimate lumbar vertebra, 

 from which, forwards, to the eleventh dorsal, there are sixteen joints between each pair of 

 vertebrae, as in the Armadillo. But this complication goes further in the Great Ant-eater, 

 for, in the penultimate lumbar vertebra, a third articular surface is developed from the under 

 and outer part of the anapophysis, which joins an articular surface on the upper and fore part 

 of the diapophysis of the last lumbar ; and this vertebra is united hi a similarly complex 

 manner with the first sacral vertebra, which would make eighteen synovial joints, in addition 

 to those at the ends of the centrum, but that those between the normal articular processes, 

 or zygapophyses, are now suppressed. The true serial homology of the processes as ' par- 

 apophyses,' developed from the fore part of the base of the neural arcn to articulate with 

 the under part of the anapophyses, is well illustrated by the vertebrae of the Great Ant- 

 eater, as hi the Megatherium, in which the true diapophyses are better developed than in the 

 Armadillos. 



Definite terms for those several processes that undergo such remarkable modifications in 

 the great Edentata will now be seen to have become indispensable ; they could scarcely be 

 rendered intelligible by the ordinary descriptive periphrases. But, by means of single sub- 

 stantive names, all their modifications can be defined, and, by adjectival inflections of those 

 names, the articular surfaces can be distinguished. 



The spines of the five sacral vertebrae are united into a continuous bony ridge ; the trans- 

 verse processes of the first join the ilium, those of the hist sacral vertebra are much thickened 

 and join the ischium. Haemal arches are developed beneath the interspaces of most of the 

 caudal vertebrae. Metapophysial tubercles are developed above the anterior zygapophyses as 

 far as these extend, viz. to the nineteenth caudal. The chief peculiarities of the rest of the 

 skeleton of the Myrmecophaga jubata have been accurately described and figured by Cuvier 

 in the ' Recherches sur les Ossemens Fossiles,' torn. v. pt. i. 



Purchased. 



Genus Bradypus. 

 2367. The skeleton of the Ai, or Three-toed Sloth (Bradypus tridactylus). 



The vertebral formula is : 9 cervical, 1 6 dorsal, 3 lumbar, 6 sacral, and 1 1 caudal. The 

 hind parts of the bodies of the second to the sixth cervical are produced backwards and under- 

 lap the fore parts of the bodies of the succeeding vertebrae. The transverse process of the atlas 

 is imperforate, but the base of the neural arch is pierced by the vertebral artery anteriorly 



3i2 



