PROFESSOR OWEN ON THE MEGATHERIUM. 
383 
development of the clavicles in the Cholcepus didachjlus, and serves, as in the Mega- 
therium, for their ligamentous attachment. In other existing Bruta the manubrium 
stern! has a broader and shorter figure, and is generally emarginate anteriorly. 
The succeeding sternal bones present the nearest resemblance to those of the Mega- 
therium in the genus Myrmecophaga, in which they are divided into two parts, each 
having articular surfaces for those on the bifurcate ends of the ossified sternal ribs ; 
but here the broad depressed portion of the sternal bone is external, the stumpy cylin- 
drical part internal or toward the thoracic cavity. The small subcubical sternebers 
of the Sloths represent the peripheral divisions only of the more complex bones in 
the Megatherium. Only the Sloths and Anteaters resemble the Megatherium in the 
small number of the lumbar vertebrae, and the Megatherium most resembles the 
Myrmecophaga in the number and complexity of the articulations between these. 
The genera Manis, Dasypus and Orycteropus have five lumbar vertebrae. 
Both the Mylodon and Scelidotherium have three lumbar vertebrae ; but these had 
coalesced with each other and with the sacrum in the skeleton of the Mylodon rohustus 
described by me*. 
The Myrmecophagoe have five sacral vertebrae as in the Megatherium : the Oryc- 
teropus has six: so likewise has the Bradypus tridactylus •. the Bradypus didactylus 
has seven sacral vertebrae; the Armadillos depart furthest from the Megatherium in 
the unusual number of vertebrae which coalesce to form the sacrum, thesie ranging 
from eight to ten in the different species. 
It would be unsafe, however, to infer that the Megatherioids were more nearly 
allied to the Anteaters than to the Sloths in respect of the structure of the sacrum 
of the Megatherium, for the Mylodon rohustus has seven sacral vertebrae, like the 
Bradypus didactylus. The posterior expansion of the sacrum and its junction with 
the ischia, so as to circumscribe the great ischiatic notch, is a character common to 
the Order Bruta. The sacrum early anchyloses with the iliac bones in the Sloths ; 
and the broad and expanded ilia of these animals, with the long and slender anterior 
parts of the ischia and pubes, circumscribing a large obturator foramen, and bound- 
ing a capacious pelvis in front by a narrow symphysis, are characters by which the 
Sloths, of all existing Edentata, offer most resemblance in their pelvis to the Mega- 
therium. 
The part of the skeleton in which the Sloths differ most from the Megatherium is 
the tail, which is so short as to be hardly visible in the entire animal ; whilst the Ant- 
eaters and Armadillos present the extensive and complex development of caudal ver- 
tebrce which characterizes the Megatherioid skeletons. In all the existing species of 
the ground-dwelling Edentate families the tail is relatively longer and more slender 
than in the Megatherium ; it is even prehensile in the Manis longicaudatus and in the 
Myrmecophaga didactyla ; but of the modifications of the terminal vertebrae on which 
MDCCCLV. 
* Description of the Skeleton of the Mylodon rohustus, 4to. 1842. 
3 F 
