352 Zoological Society. 



slightly marked, or the former wanting, the supratemporal process 

 rising obliquely ; pterygoid bones compressed and simple ; crota- 

 phite impression extending to very near the occipital ridge ; tympa- 

 nic bone well-developed, inflated ; lower jaw with its inferior outline 

 concave posteriorly, its condyle elevated ; teeth ^, simple, rounded, 

 the anterior ones similar, small in the upper jaw. 



A. GULARis. j4'i d dos brute. 



A broad patch of soft yellow hair between the shoulders, and a 

 black line running through it down the back ; the upper anterior 

 molars proportionally larger, and the second less, than in the fol- 

 lowing species; the occiput again affords us a very good distinction, 

 as it is much wider and not so deep as in the following species, and 

 the foramen magnum not so large. Two skulls in the British Museum 

 present these characters, and evidently belong to adult, probably 

 aged, individuals ; that of the skeleton, also from Bolivia, seems 

 referable to the other species. 



A. MARMORATUS. 



Fur everywhere more or less lengthened, no yellow spots, dorsal 

 line grey brown ; anterior upper molars very small, the next rather 

 larger than those which follow ; occiput deeper and narrower than 

 in the preceding species, its foramen larger. 



The A. Blainvillii is not distinguishable by external markings, and 

 the skulls bearing that name in the Museum collection all present a 

 general robustness, such as age and sex might very probably occasion. 

 One of them, which, from retaining some of the sutures, seems to be 

 younger than the others, has the frontal bones less swollen, and the 

 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. flaccidus may be only a local variety, the skulls not being 

 veiy clearly distinguishable, for there are not 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 inflated ; malar bone with its 



