56 



tions of the posterior plate and anterior cube are less unequal. The articulations 

 on the upper angles of the cube are still distinct from those on the corresponding 

 angles of the plate ; but the lower ones of the plate and cube are confluent on 

 each side, forming an oblong surface, slightly contracted in the middle, and se- 

 parated by deep and narrow depressions from the triangular median surface for 

 the articulation of the ninth sternal bone. 



Comparison of the true Vertebra. 



The vertebrae of the skeleton of the Mylodon here described would appear to 

 have been discovered in their natural relative position, for they were numbered 

 consecutively from the atlas to the twenty-third vertebra, the body of which was 

 separated from the above-described anchylosed neural arch. The exact adjust- 

 ment of the articulations of the vertebrae, arranged according to these num- 

 bers, left no doubt as to their accuracy : and, whilst the indications of the trans- 

 verse processes showed that five cervical vertebrae succeeded the atlas and den- 

 tata, the number of the ribs equally established the correctness of assigning the 

 remaining sixteen vertebrae to the dorsal region. 



Few as are the striking or important modifications of the cervical vertebrae 

 in the Mammaha, and closely as those of the Mylodon adhere to the com- 

 mon condition of this part of the spine, certain features of resemblance to the 

 corresponding part of the vertebral column in the Sloths may nevertheless be 

 discerned, as, for example, the great relative breadth of the atlas compared 

 with the skull, and the shortness of the dentata. The most remarkable pecu- 

 liarity of the cervical region is presented, as is well known, by the three-toed 

 species of Sloth, which has the exceptional number of nine vertebra anterior to 

 that which supports the first true rib. The Mylodon, in conforming, like the 

 Megatherium, to the ordinary mammalian number of seven cervical vertebrae, 

 agrees with the two-toed Sloth. And not only in the number, but in the forms 

 and proportions of the spinous processes, the Unau resembles the Mylodon more 

 than does the more anomalous three-toed Sloth. 



The atlas of both species of Sloth has a distinct tubercular rudiment of a spine 

 on the neural arch ; but the Unau's atlas further differs from the Mylodon's in 



