276 University of California Publications in Geology [Vol. 10 



separate phalanges, together with the location of principal vertical 

 movement between the first phalanx and the metatarsal in Mylodon, 

 may be in part conducive to such co-ossification. It is more probable, 

 however, that the co-ossification of the two phalanges in the genera 

 cited above is to be correlated with the large ungual of the median 

 digit, which prevails in these forms and requires a rigid support, while 

 in Mylodon the relatively much smaller third ungual is accompanied 

 by a more loosely articulating digit. Fusion of the two phalanges 

 in M. harlani may depend upon the age of the individual. 



In the identification of the foot material of Mylodon garmani, 

 Dr. G. M. Allen, 11 who of necessity has followed Owen's determinations 

 for M. roiutus, has assigned to the second digit of the manus of 

 the former species an element which is evidently identical with the 

 structure under consideration. Allen 12 states : 



The phalanx next, proximal to this [referring to the ungual of digit 2] is 

 short, 53 mm. long, with two rounded facets anteriorly, and a broadly concave 

 facet posteriorly. On each side of the latter, at its ventral corners, is a small 

 squarish facet which evidently articulated with a sesomoid. 



The character of the proximal and distal ends and the length of 

 tlie phalanx establish its identity as the homologue of the co-ossified 

 first and second phalanges in the third digit of the pes of M. harlani. 



Characteristic features exhibited by the ungual of digit 2, manus, 

 and digit 3, pes, of M. robust us as interpreted by Owen may be 

 tabulated as follows : 



Ungual, Digit 2, Manus Ungual, Digit 3, Pes 



(1) One half the dimensions of un- (1) Judging from figures, ungual 

 gual of digit 3, manus. much larger than that of digit 2, 



manus, and approaching terminal pha- 

 lanx of digit 3 in size. 



(2) Backward production of the (2) In extreme extension, long axis 

 upper part of the proximal joint per- may be parallel with that of second 

 mits claw-bone to be bent only down- phalanx. 



wards. 



(3) Broad, rough, quadrilateral base (■'!) Flat, rough, oval surface at the 

 of claw-sheath. proximal half of base of claw. 



(4) Claw-process conical, nearly (4) Claw-process forms chief part 

 straight, with an oblique base which of ungual, conical, slightly deflected, 

 gives under only half the length of inclined inwards. Under surface, ow- 

 the upper part. ing to oblique line from which sides 



of ungual sheath arise, is less than 

 half the length of the upper surface. 



11 Allen, G. M., A new Mylodon, Mem. Mus. Comp. Zoiil. Harv. Coll., vol. 40, 

 pp. 319-346, 4 pis., 1913. 

 1= Ibid., p. 336. 



