198 ANNALS NEW YORK ACADEMY OF SCIENCES 



Calcaneuin (Plate IX, Fig. -t). — The well preserved calcaneum is 

 quite large with a widely expanded free portion. The border is exten- 

 sively roughened for tendon attachment.- The facet for the astragulus is 

 about 5 mm. posterior to the cuboid facet, showing that the astragulus 

 must be of a long-necked type (no astragulus was obtained). Greatest 

 length, 41 mm. ; width of expanded portion, 30 mm. ; thickness of ex- 

 panded portion, 4.5 mm. ; transverse width of cuboid articulation, 15 

 mm. ; greatest width of calcaneum at anterior end, 19.5 mm. 



Vertehrce (Plate IX, Figs. 5-7). — Three vertebrse were obtained — the 

 axis, a dorsal and a caudal vertebra. 



The axis has a fairly high, wide, keel-shaped neural spine and a short 

 thick odontoid process. The neural canal is very large. Height from 

 bottom of centrum to top of neural spine, 34 mm. ; length of centrum 

 from end of odontoid, 24 mm. ; height of neural spine from roof of neural 

 canal, 13 mm. 



The dorsal vertebra bears a long spinous process set at a low angle with 

 the vertebral column. The transverse processes are short ; the ribs articu- 

 late, the tubercle with the transverse process, the head, partly with the 

 centrum of the anterior vertebra, but to a greater extent with a facet on 

 the wall of the neural canal of the posterior vertebra. The centrum is 

 subtriangular in cross-section. Length of spinous process from plane of 

 anterior margin of transverse processes, 27 mm. ; height of spinous process 

 above roof of neural canal, 12 mm. ; length of centrum, 14 mm. 



The caudal vertebra is peculiar in having a depressed, flattened appear- 

 ance with wide transverse processes. The spinous process is low (dis- 

 torted?) and the neural canal very small. There are no other noticeable 

 projections from the body of the vertebra. This is a condition very 

 closely approximating that found in ChoJcepus of the Bradypodidse. 

 Width across transverse processes, 38 mm. ; vertical thickness of centrum, 

 posteriori}^, 8 mm.; lateral width of centrum (articulating surface), 

 posteriorly, 17 mm. 



REMABKS 



Acratocnus presents characters that sharply mark it off from any hith- 

 erto described forms. It seems to have no close direct affinities with any 

 of the Megalonychidae, but relationship, somewhat removed, is shown to 

 several genera. Compared with the Hapalops-Eucholceops group of the 

 Santa Cruz formation of Patagonia, a distant relationship may be as- 

 sumed on the basis of the comparative lightness of limb bones of the 

 Hapalops-JSucholoeops type. Acratocnus is even more slender limbed 

 than the Santa Cruz sloths, the femur of this genus, for example, being 

 almost as long as that of Hapalops rutimeyeri but of less than half its 

 breadth. However, the Santa Cruz animals are the lightest limbed, most 



