72 



sent off from the caudal aorta. In the third and succeeding vertebrse to the 

 ninth inclusive, the spinal artery perforates instead of indenting the angle or 

 ridge dividing the under from the lateral surface of the centrum* : in the tenth, 

 eleventh, and twelfth vertebra this angle is again deeply indented. 



The series of caudal vertebrse in their easiest and most natural juxtaposition 

 forms a gentle curve with the concavity directed upwards and backwards ; and 

 the extremity, in the attitude in which the skeleton of the Mylodon is repre- 

 sented in the first Plate, rests by its under surface on the ground, plainly indi- 

 cating that this robust tail aided in supporting the massive pelvis. 



Comparison of the Pelvis. 



The sacrum in the Sloths is broad and also unusually long ; it includes in the 

 Unau seven vertebrae, corresponding with the number of the true sacral ver- 

 tebrje in the Mylodon ; the Ai has six sacral vertebrae. The anterior of these, in 

 both species of Sloth, are anchylosed at an early period to the broad and ex- 

 panded iUa, and the posterior vertebrse extend outwards to coalesce with the 

 ischia. Both ischial and pubic bones are long and slender ; they circumscribe 

 large obturator foramina, and bound by a narrow symphysis a very capacious 

 pelvis. The existing Sloths thus offer a very close approximation to the Mylodon 

 by the repetition of the most essential characters of its pelvis. They differ in the 

 absence or feeble development of the spines and ridges which relate to the size 

 and force of the muscles attached to tlie pelvis. 



The Ant-eaters have five sacral vertebrse ; the Pangolins three ; the Orycterope 

 six. In the great Ant-eater the sacral spines coalesce into a long crest of bone, 

 and in an aged specimen I have seen the last lumbar vertebra anchylosed to the 

 sacrum, botl which are interesting features of resemblance to the extinct Mylo- 

 don ; but the iliac bones of the Ant-eater are long and narrow, and the pelvis 

 has the usual laterally-contracted form in quadrupeds. The pelvis is more ex- 

 panded in the Manis, but the ilia are short, thick, straight, prismatic bones : 

 they have the same form in the Orycterope. In all the Edentata the great 

 ischiadic notch is converted into a foramen by the junction of the sacrum with 



* The outlets of these canals are seen in Plate I. 



