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 4. flaccidus may be only a local variety, the skulls not being 
very 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 palzontologist 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. 
Mereatuerivum, Cuvier. 
Intermaxillary bones lengthened and prominent ; postorbital pro- 
cess lengthened and drawn out, but not inflated; 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 cementum being thickened on the anterior and posterior surfaces ; 
the posterior upper one small. 
M. Cuvier. 
Dr. Lund figures a tooth having the characters of this well-known 
genus, but of smaller size, under the name of Megatherium Laurillardi. 
Meeatonyx, Jefferson. 
General cranial characters unknown ; teeth as subelliptical, with 
a ridge on the inner side. 
M. Jeffersonii. 
Mytopon, Owen. 
Intermaxillary bones small (lost in the skeleton) ; postorbital pro- 
cess but little developed, thick; malar bone with the frontal process 
indicated by a slight angle, the zygomatic well-developed, touching 
No. CCXXXII.—Procerpinés or THE ZooLoeicat Society. 
