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University of California Publications in Geology [Vol. 10 



first phalanx of digit 2, manus of M. harlani, in the much shorter 

 anteroposterior diameter. This is due to the flatness of the two artic- 

 ulating condyles on the distal face for phalanx 2, and to the more 

 shallow proximal concavity. The latter concavity, which receives the 

 convex distal carina of the third metatarsal, narrows below. Along 

 almost the entire inner side of this concavity is an offset which widens 

 below, while at the lower end of the outer side a small offset is also 

 present. In M. robustus Owen describes the proximal face of the 

 phalanx as "concave, adapted to the upper two-thirds of the convex 

 trochlea of the metatarsal bone, and widening as it descends. ' ' Below 

 each offset and on the ventral surface are two small facets for the 

 sesamoids. Between these the ventral margin of the phalanx is broadly 

 concave. 



As interpreted by Owen, the characteristic features of the second 

 phalanx, digit 3, of the pes of M. robustus and the corresponding 

 phalanx of digit 2, manus, may be briefly tabulated as follows : 



Phalanx 2, Digit 2, Manus 



(1) Long in proportion to its 

 breadth, which is greatest in the ver- 

 tical diameter of its base. 



(2) Proximal articulation concave 

 vertically, traversed by a median ob- 

 tuse ridge. 



(3) Distal articulation describes a 

 semicircle. 



Phalanx 2, Digit 3, Pes 



(1) Anteroposterior diameter ex- 

 ceeds by one-eighth part the vertical 

 diameter, and by one-third part the 

 transverse diameter. 



(2) Proximal articulation consists 

 of two vertical concavities separated 

 by a median ridge. Upper extremity 

 of latter ridge more produced than 

 lower. 



(3) Convex borders of distal troch- 

 lea describe two-thirds of a circle. 



In dimensions and in structure of distal trochlea, the second 

 phalanx of digit 3, pes of Mylodon harlani, differs from the corre- 

 sponding phalanx of the pes of M. robustus and agrees more closely 

 in these characters with phalanx 2, digit 2, manus of the latter species. 

 This difference from Owen's interpretation is distinctly brought out 

 also by a comparison with his figures (see figs. 1 and 2, also Owen, 

 pi. 15). The depth of the proximal end of the second phalanx may 

 be slightly less than or may exceed the total length of the phalanx in 

 M. harlani. The second phalanx is often only half as large as the 

 corresponding phalanx in the second digit of the manus. The proxi- 

 mal face is broad at the base and narrows dorsally to a posteriorly 

 projecting process approximately one-half of the basal width. Some 



s Owen, op. ext., p. 123. 



