105 



is overtopped by an obtuse protuberance, impeding any upward retraction 

 of the claw, and well-illustrating the argument bv which Cuvier esta- 

 blished the true affinities of the allied genus Megalonyx. The present 

 phalanx is, however, less compressed and less incurved than in that 

 genus. The osseous sheath for the claw is developed only at the under 

 part of the bone, where it presents the form of a thick flat plate, with 

 the margin obliquely bevelled oft", and with a vertical ridge of bone 

 attached lengthwise to the middle of its under surface. This process 

 doubtless served for the insertion of a very powerful flexor tendon. 



Megatherioid Fossils. 



506. The body of a vertebra, probably the last cervical, of a Megatherioid 

 animal ; it exhibits the great width of the spinal canal which characterizes 

 this extinct family of Edentate quadrupeds, and presents two tuberosities, 

 one on each side of the posterior part of the lower surface. 



From South America. Presented by Charles Darwin, Esq., F.R.S. 



50/. A dorsal vertebra of a large Megatherioid quadruped, which well displays 

 the characteristic capacity of the spinal canal, and the third posterior 

 articular process situated in the interspace of the two normal processes. 

 From the tertiary deposits of Buenos Ayres. Purchased. 



508. The body of a dorsal vertebra of the same species of Megatherioid 



animal. 



From the same stratum and locality. Purchased. 



509. The inferior bony arch of a caudal vertebra of a Mylodon or Megalonyx ; 



it is formed by the distal confluence of the two haemapophysial elements, 

 each of which upon its proximal end or base supports two distinct arti- 

 cular surfaces. 



From the tertiary deposits at Buenos Ayres. Purchased. 



510. The body and anchylosed posterior crura of thehyoid bone of probably a 

 Megatherioid animal, as large as the Mylodon ; it differs from the corre- 

 sponding parts of the hyoidean apparatus in the Mylodon robustus, in 



p 



