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bone in advance of the expanded part giving articulation to the first rib. This pro- 

 longation, which is not present in the Bradypus tridactylus, relates to the complete 

 development of the clavicles in the Cholcepus didactylus, and serves, as in the Mega- 

 therium, for their ligamentous attachment. In other existing Bruta the manubrium 

 sterni 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 vertebras ; but these had 

 coalesced with each other and with the sacrum in the skeleton of the Mylodon robustus 

 described by me*. 



The Myrmecophaga; 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, these 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 robustus 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 bounding 

 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 Megatherium. 



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- 

 tebrae 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 Myr- 

 mecophaga didactyla ; but of the modifications of the terminal vertebrae on which that 

 power depends there is no trace in the Megatherium any more than in the Mylodon. 

 * Description of the Skeleton of the Mylodon robustus, 4to. 1842. 



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