38 



A comparison of the skull of the Mylodon with that of the Megalon5'x would 

 doubtless be replete with interest, since many intermediate approximations to 

 the Megatherium might be anticipated : but the requisite materials are still 

 wanting. The only part of the skull of the Megalonyx hitherto determined 

 upon sufBcient grounds for such a comparison, is the mutilated lower jaw, de- 

 scribed and figured in the 'Fossil Mammalia of the Beagle*.' As compared 

 with the Mylodon, the rami of this lower jaw, instead of being straight and 

 parallel, recede from each other in the molar region by a slight outward cur- 

 vature, and besides the generic differences in the form of the teeth, the two 

 series slightly converge anteriorly in the Megalonyx, instead of diverging, as in 

 the Mylodon ; the rami of the jaw also meet anteriorly with a more contracted 

 curve. 



The extinct Megatherioid animal, of which, after thcMylodon and Megatherium, 

 the most complete cranium has hitherto been obtained, is the Scelidotherium f. 

 This presents the before-mentioned essential characters of the Sloth's skull, but 

 with the mylodontal modifications which are manifested in the complete zygoma 

 and trifurcate malar bone. The occipital plane differs from that of the Mylodon 

 and Sloths in being vertical ; and the palate is narrower, the rami of the lower 

 jaw more convergent as well as relatively shallower anteriorly, than in the My- 

 lodon. But the Scelidothere, in the general form and proportions of the skull, 

 in the depth of the molar portion of the jaws, in the contour of the lower 

 margin, and in the extent and shape of the angle of the lower jaw, resembles the 

 Mylodon, not the Megatherium. Its chief approximations to the Megatherium 

 are made by the absence of the diastema between the first and second molars in 

 the upper jaw, and by the contraction of the palate, which, however, is less 

 marked in the Scelidothere. A valuable fact is yielded by the skull of the Sce- 

 lidothere, from which these observations were taken, in the persistence of the 

 stylo-hyal bone, cemented by the matrix to its natural place of articulation, viz. 

 the cavity in the mastoid bone already described. By this I have been enabled 

 to recognise the corresponding bone in the collection of remains of both the 

 Megatherium and Mylodon in the Museum of the College. 



* Part IV. 4to, p. 99, pi. 29. 



t Fossil Mammalia of the Beagle, p. 73, pi. 21, 22, and 23. 



