PEOFESSOE OWEN ON THE SCELIDOTHEEE. 
109 
pital plane is more vertical than in the Megathere, and still more so than in the Mylodon 
or in the existing Sloths. In the minor development of the mastoid as compared with 
the paroccipital, the Scelidothere resembles the Mylodon more than it does the Mega- 
there : it cUffers from both in the much smaller sm’face of articulation for the stylo- 
hyal ; and, apparently, by the want of junction of the malar with the squamosal. The 
lower jaw, whilst it resembles that of the Mylodon in the form of the ascending ramus 
and of the dentigerous part, differs in the unusual prolongation of the long and slender 
symphysis, and in the smaller size of the mental foramina : the intermediate character 
between Mylodon and Megathere is well illustrated in the mandible of the Scelido- 
there. The dentition of the Scelidothere differs from that of the Megathere, and 
resembles that of the Mylodon in the simple concavity of the grinding surface ; but it 
differs from the dentition of the Mylodon in the first upper molar not being divided by 
a disproportionate interspace from the rest, and by the more unequal sides of the tri- 
angular grinding surface. The second molar, in the Mylodon, presents an elhptical 
transverse section instead of the reniform or subtriangular shape. The third and fourth 
molars of the Scelidothere are more compressed than in the Mylodon, and their long 
axis is from before backwards, instead of being transverse. The fifth molar is relatively 
smaller than in the Mylodon, approaching to that in the Megathere. In the lower 
jaw of the Scelidothere the difierences in the form of the teeth are equally manifest, 
especially in the prismatic form of the first molar : it is elliptical in the Mylodon. The 
last molar of the Mylodon rolmstus has the second lobe larger than the first : in the Mylodon 
Darwinii the proportions of the lobes more resemble those in the Scelidothere. The two 
middle teeth differ more markedly from the corresponding ones in any of the species of 
Mylodon : the second molar in M. rohustus has a triangular transverse section with the 
base turned inwards and indented ; the third molar is subquadrate. 
The retention of the mylodontal character of dentition in three forms of the genus, 
with modifications not surpassing those of specific value, renders it probable that other 
species of Scelidotlierhim may have existed besides the one under description. The 
generic distinction of Scelidotlierium from Mylodon is strengthened by the additional 
characters which the complete crania in the British Museum have brought to fight. 
In conclusion I may remark, that, as our knowledge of the great Megatherioid animals 
increases, the definition of their distinctive characters demands more extended comparison 
of particulars : hence in each successive attempt at a restoration of these truly remark- 
able extinct South American quadrupeds, there results a description of details which 
might else seem prolix and uncalled-for. 
I have only to add, that the allotment of the Government Grant for dravAngs of rare 
or nondescript fossils has enabled me to bring before the Society without delay the 
present account, with adequate illustrations of the entire skull of the Scelidotlierium 
le/ptocephalum. 
MDCCCLVII. 
Q 
