34 



the mylodontal proportions ; in no other does it ascend above the zygoma 

 into the temporal region, or descend below the alveolar tract. And the simi- 

 larities of cranial organization do not end here : the complicated malar bone is 

 associated, in the Sloths, with a terminal position of the great anterior and 

 posterior orifices of the cranium ; with terminal occipital condyles ; with a sloping 

 occipital region ; with a smooth and crestless parietal tract ; and with a muzzle 

 almost as short, thick, and abruptly truncated as in the Mylodon robustus. 



The cranial division of the skull is relatively as great in the Sloths as in the 

 Mylodon ; and the actual capacity of the cerebral cavity is masked by a similar 

 expansion of the air-cells which almost everywhere surround that cavity, and 

 raise the outer plate of its bony parietes above the vitreous table. 



The occipital bone presents the same expanded proportions, the same broad 

 and flat basilar plate ; and the anterior condyloid foramina are of conspicuous 

 size. The tympanic bone nearly completes a circular frame for the ear-drum, to 

 which function it is hmited, and long remains separate. The detached and lost 

 tympanic bones of the Mylodon have been evidently restricted to the same office. 

 The temporal fossa is long and large, and is continued freely into the orbit: the 

 squamous plate of the temporal scarcely rises half-way towards the upper 

 boundary of the fossa, and terminates by an almost horizontal margin. The 

 supraorbital boundary, continued from an obtuse postorbital process, is almost 

 lateral. The single lachrymal foramen is perforated exclusively in the lachrymal 

 bone. The large quadrilateral nasals become sometimes confluent in the Sloths. 

 The intermaxillary bones are edentulous, and without ascending process ; they 

 are almost of rudimental proportions in the Unau, and in the Ai are represented 

 by a single small triangular plate, bounding by its base a triangular cleft at the 

 anterior part of the maxillaries, and thus defining the incisive foramen. We 

 may thence infer that the anterior palatine cleft was similarly inclosed in the 

 Mylodon. The correspondence here pursued is not less closely marked in the 

 lower jaw ; the rami of which, in the Sloths, enlarge and branch out posteriorly 

 into a coronoid, a condyloid, and a long and deep angular process, and are an- 

 chylosed anteriorly at a broad sloping symphysis. Only in the genus Bradypus, 

 amongst existing quadrupeds, do the alveoli of both jaws correspond in number, 

 simplicity, relative depth, and position with those of the genus Mylodon. The 

 still more important agreement between these existing and extinct Bruta in the 



