209 



lower jaw with its angular process as much produced as in those 

 labelled marmoratus, though deeper, but not so deep as in the others. 



The A. jlaccidus may be only a local variety, the skulls not being 

 very clearly distinguishable, for there are itot two between which 

 some individual peculiarities may not be traced. 



The skull to which the name problematicus is given is evidently 

 young, having all its sutures well-marked, and in the absence of the 

 fur cannot be safely looked upon as the type of a species. It agrees 

 with the others in the character of the occiput, which distinguishes 

 them all from the A. gularis, as well as from the Bradypus crinitus. 

 The palaeontologist is well aware of the uncertainty of establishing 

 species upon trivial details of form, although slight distinctions are 

 in some cases known to afford a true indication : the skulls of the 

 Three-toed Sloths vary greatly, and all present a coarse, rough-hewn 

 appearance which must detract from our confidence in little differ- 

 ences of detail. With regard to the lower jaw, they certainly do not 

 present differences so strikingly characteristic as those upon which 

 the species of Mylodon are established. 



Megatherium, Cuvier. 



Intermaxillary bones lengthened and prominent ; postorbital pro- 

 cess lengthened and drawn out, but not mflated ; malar bone with its 

 frontal and zygomatic processes well-developed, the latter attached 

 firmly to the zygoma ; the supratemporal process rising obliquely ; 

 pterygoid bones compressed, and not inflated ; crotaphite impression 

 approaching near to the occipital ridge ; tympanic bone attached, 

 small, and not inflated ; (immediately in front of the circular facet 

 for the stylohyal bone there descends a strong process, which may 

 probably belong to the tympanic bone and form a portion of a vaginal 

 process ;) lower jaw produced in front, deepened in the middle by the 

 extensive implantation of the molars, the condyle much elevated ; 

 teeth ^^, quadrate, grooved transversely on the crown when worn, 



the csementum being thickened on the anterior and posterior surfaces ; 

 the posterior upper one small. 



M. CUVIERI. 



Dr. Lund figures a tooth having the characters of this well-known 

 genus, but of smaller size, under the name oi Megathei-iumLaurillardi . 



Megalonyx, Jefferson. 



General cranial characters unknown ; teeth '^^, subelliptical, with 

 a ridge on the inner side. 

 M. Jeffersonii, 



Mylodon, Owen. 



Intermaxillary bones small (lost in the skeleton) ; postorbital pro- 

 cess but little developed, thick ; malar bone vni\\ the frontal process 

 indicated by a slight angle, the zygomatic well-developed, touching 



No. CCXXXIT. — Proceedings of the Zoological Society. 



