39 



the depression at the anterior end of the maxillary bone for the recep- 

 tion of the intermaxillary. The upper part of this instructive fragment 

 is broken away and water- worn, exposing the pulp- cavities of the four 

 anterior teeth of the left side : the contour and relative size of the crowns 

 of the entire series of teeth on this side are demonstrated by a transverse 

 section. 



220. The longitudinal vertical section of the right molar series, removed from 



the preceding specimen : it shows also the great vertical extent and even 

 surface of the superior maxillary bone behind the orbit. 



The portion of skull which has afforded the two preceding prepara- 

 tions was discovered at Pimta Alta, Bahia Blanca, Patagonia, 

 and was presented to the College by Charles Darwin, Esq. 

 F.R.S. 



221. The posterior part of the skull of the Megatherium : the occipital con- 



dyles project outwards and backwards from the foramen magnum, and 

 form the posterior termination of the skull ; the occipital region slopes 

 upwards and forwards from the foramen, and is excavated by two deep 

 depressions, separated from each other by a strong median crest, and se- 

 parated by a thicker arched ridge from the upper boundary of the occipital 

 region, which has here been broken away ; external to these cavities 

 there is a narrower depression, probably for the origin of the digastric 

 muscle : the anterior condyloid foramen is absolutely smaller than in the 

 Mylodon, indicating a much smaller relative development of the tongue : 

 the mastoid part of the temporal is excavated beneath by a moderately 

 large and smooth elliptical cavity, for the reception of the head of the 

 styloid or stylo-hyoid bone : the basilar portion of the sphenoid, which 

 intervenes between these cavities, is dilated by air-cells : the carotid 

 canal runs on each side between these bullae ossese and the petrous 

 bone, and curves inwards, indenting the upper part of the body of the 

 sphenoid, which forms the broad and shallow sella turcica. The posterior 

 part of the cranial cavity is broader than it is high, and is small in pro- 

 portion to the size of the animal. The vertical diameter of the cranial 

 cavity is four inches, eight lines ; its transverse diameter, which is 



