740 
PROFESSOR OWEN ON THE MEGATHERIUM. 
Order Bruta {Edentata, Cuv.). 
The exogenous processes of the vertebrae present their greatest and most complex 
developments in this order, but by no means with that degree of uniformity which 
renders their modifications characteristic of some of the preceding Orders. The two 
extremes in the degree of development are, in fact, presented by the much-diversified 
families of the present extraordinary group of mammals. 
The spinous processes are almost obsolete in a great part of the unusually long 
dorso-lumbar region of the Two-toed Sloth {Bradypus didactylus), and the rnetapo- 
physes would be hardly recognizable if their modifications had not been previously 
traced in other mammals. The first indication of them is seen in the penultimate 
dorsal vertebra, where tliey appear as angular productions from the upper and fore- 
part of the diapophyses : in the last dorsal they have advanced clear of the diapo- 
physis to the outer side of the prozygapophysis, the articular surface of which begins 
to ascend upon them ; and they continue, as low and long tuberosities, above the pro- 
zygapophyses, throughout the lumbar region. In the sacral vertebrae they are obso- 
lete. No anapophyses are developed in this species of Sloth*. 
* In assigning, with Cuvier, this formula to the dorsal region of the spine, I have been guided by characters 
which have appeared to me more demonstrative of the homology of the eighth and ninth vertebrae of the Ai, 
with the last two cervicals of other mammals, than the sometimes persistent freedom of their short pleurapo- 
physes are of their homology with the first two dorsals. The penultimate cervical vertebra, for example, has 
a well-marked and constant character in most mammals in the greater antero-posterior extent of its anchylosed 
pleurapophysis ; and I find this character repeated in the eighth vertebra, counting from the atlas, in the Three- 
toed Sloth. Subjoined are the chief particulars which are noticeable in the cervical region of the skeleton of the 
mature specimen of that species in the Museum of the Royal College of Surgeons. 
The hind-part of the bodies of the second to the sixth cervical vertebra? inclusive, are produced backwards and 
underlap the fore-part of the body 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 and by the cervical nerve posteriorly. 
The spines of the cervical vertebrae are moderately and more equally developed than in other mammals ; that of 
the dentata being little larger than the rest. The pleurapophysial part of the transverse process of the eighth 
cervical is anchylosed, and is more extended antero-posteriorly than in the preceding cervicals, in which respects 
it resembles the sixth cervical vertebra in ordinary quadrupeds. The pleurapophysial part of the transverse 
process of the ninth cervical is free, and is more extended in the direction of its length, but is very short com- 
pared with the homologous part of the succeeding vertebra. The slender neck and head of this little rib joining 
the fore-part of its centrum occasions the perforated character of the ninth, as in the antecedent cervical ver- 
tebrse. In the fourth cervical, however, the vertebral artery perforates the right transverse process, but only 
grooves the left on its anterior part. The transverse processes of the second and third cervical vertebrae are both 
imperforate. 
The homology of the vertebra succeeding the ninth with the first dorsal of the Unau and of other mammals, 
is demonstrated by the junction of its rib with the manubrium by the superadded hsemapophysial element, which 
is here, however, ossified and anchylosed with both its pleurapophysis and the manubrium. Nine pairs of ribs 
directly articulate to the sternum, which consists of eight bones ; these are compressed and progressively in- 
crease in depth. The hinder ones are divided into a larger posterior and a smaller anterior part, between which 
are four articulated facets on each side for the bifurcated extremities of two of the ossified hsemapophyses f. 
t Proceedings of the Zoological Society, 1849, p. 146. 
