406 



ANATOMY OF VERTEBRATES. 



Skull of Megatherium, one-flfteenth nat. size. 



The facial part of the skull in the Sloth is as remarkable for its 

 shortness as in the Anteater for its length. In the Ai, fig. 273, 

 the interparietal coalesces with the superoccipital, 3, before the 

 exoccipitals unite Avith the super- and basi-occipitals. The malar 

 bone, 26, is freely suspended by its anterior attachment to the 

 maxillary and frontal, and bifurcates behind ; one division, a, ex- 

 tending downward, outside the lower jaw, the other, h, ascending 



above the free termination of 

 ^^^ the zygomatic process of the 



squamosal, 27. The premax- 

 illary is single and edentu- 

 lous, being represented only 

 by its palatal portion com- 

 pleting the maxillary arch, 

 but not sending any pro- 

 cesses uj^ward to the nasals. 

 Within the cranium there is 

 no bony tentorium : the two 

 divisions of the meatus in- 

 ternus commence separately 

 upon the exterior of the petrosal, which is not impressed by a 

 cerebellar fossa. The depression receiving the natiform protu- 

 berance of the cerebellum is formed chiefly by the squamosal. 

 The walls of the rhinencephalic fossa are entirely surrounded by 

 the olfactory chamber, which extends above into the frontal and 

 beneath into the sphenoidal sinuses. A well-marked vascular fora- 

 men leads downward from the partition between the rhinencephalic 

 and prosencephalic chambers. The rough exterior part of the 

 petrosal forms, as it were, the border of a capsule to the tympanic : 

 the fossa for the stylohyal is well marked at the back part of the 

 border. The pterygoid forms a large quadrate vertical plate. The 

 bony septum narium terminates half-way from the large vertical 

 external nostril. There is a small imperforate lacrymal : the ant- 

 orbital hole is wanting. In the Unau {Bradypus didactijlus)t\\Q 

 lacrymal is pierced external to the orbit. In the Megathere the 

 foramen is at the orbital margin, and there is a large antorbital 

 foramen : the premaxillaries, though edentulous, are more produced 

 than in existing Sloths, and there is a corresponding production 

 of the symphysial part of the lower jaw, which is grooved above, 

 as in the Anteaters, for the support during its pro- and re-tractile 

 movements of a long tongue, prehensile in the ISIegathere as in 

 the Giraffe, in reference to the smaller branches of the trees 

 yielding food to the extinct giant. Behind the symphysis the 



