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XXVIL On the Megatherium (Megatherium Americanum, Cuvier awrZBLUMENBACH). 
Part III. — The Shull. By Professor Owen, F.R.S. ^c. 
Received November 8, 1850 — Read May 8, 1851. 
/ 
The vertebral segments which coalesce more or less completely to form the 
important and characteristic part of the skeleton called the ‘ skull,’ constitute, in the 
Megatherium, one which is remarkable for its small relative size to the rest of the 
body, for the slight diminution of the transverse diameter from the occiput forward 
to the nasal bones, for the length and strength of the ascending and descending pro- 
cesses from the orbital end of the zygomatic arch, and for the peculiar depth of the 
lower jaw, especially at its middle part, where it lodges the molar teeth. 
The widest part of the long and narrow cranium is formed by the zygomatic 
arches, Plate XXII. fig. 2, 26, 27, and the deepest part by the dentigerous portion of 
the lower jaw, Plate XXL fig. 2, d. 
In the skulls and portions of skulls of the Megatherium which have come under 
my observation, all the cranial and most of the facial sutures, save those that unite 
the tympanic to the rest of the temporal bone, had been obliterated, and the origin- 
ally complex assemblage of bones forming the cranium and upper jaw had been 
reduced to a continuous whole. 
Viewed from behind (Plate XXIII. i), the skull shows the unusual degree to which 
the pterygoid, 24, palatine, and maxillary portions descend below the level of the true 
‘ basis cranii ’ ( i ) : in consequence of which the foramen magnum (o) and occipital 
condyles (2) appear to be situated in the upper half of the direct back view. 
The basioccipital (1) is a broad depressed plate with a thin, smoothly rounded, 
concave posterior border : it slightly increases in thickness as it advances forward 
to blend with the basisphenoid (Plate XXIV. 5), but developes on each side a rough 
low triangular protuberance, z, for muscular attachments, anterior to the occi- 
pital condyles. In a side view (Plate XXL fig. 1), these condyles (2) form the 
most prominent parts of the occipital region, which, as it rises above them, slopes 
forward, giving a low character of intelligence to the cranium : the plane of the 
posterior surface of the skull forms with the basal plane an angle of 105°. Each 
condyle (Plate XXIII. fig'. 1 , 2, 2) is a conv’^exity of a subtriangular form, with the base 
straight and the sides curving to an obtuse apex: the extent of their convex curva- 
ture in the vertical or antero-posterior direction (Plate XXL fig. 1,2) equals that of a 
semicircle, and indicates that the Megatherium possessed considerable freedom and 
extent of motion of the head. The parallel bases of the condyles are turned toward 
each other and flank the sides of the foramen magnum, Plate XXIII. fig. 1, o. This 
MDCCCLVI. 4 F 
