66 



The shaft of the femur is more or less flattened in all the species of Bruta ; but the 

 Orycteropus and Armadillos, in which this character is conspicuous, differ, like the Rhino- 

 ceros, from the Megatherium in having the third trochanter. This process is not present 

 in the Pangolins (Manis), Anteaters (Myrmecophaga) or Sloths (Bradijpus) ; but in all 

 these genera the femur is relatively longer and more slender than in the Megatherium, 

 and only the Sloths amongst existing Mammals, not marine, repeat the remarkable mega- 

 therioid character of the absence of a medullary cavity in the shaft of the femur. 



The general characters of the femur of the Megatherium are most closely repeated in 

 that of the Mylodon* and Scelidotheriumf. In these genera, however, instead of the 

 shallow notch, there is a deep and prolonged fossa for the ligamentum teres on the 

 middle of the hind border of the head of the bone. The post-trochanterian depression 

 is relatively larger in the Mylodon, the small trochanter is relatively less and higher 

 placed ; the whole femur is longer in proportion to its breadth, and the distal expansion 

 is relatively less. Here, also, the articular surfaces offer a well-marked character of 

 distinction ; the rotular articular surface is continuous with that of both condyles, and 

 unites them anteriorly. 



From the cast of a distal epiphysis, transmitted by Dr. Harlan to the Museum of 

 the Royal College of Surgeons, London, as of the Megalonyx, it would appear that this 

 extinct megatherioid offered a third modification of the knee-joint, the rotular surface 

 being distinct from those of both condyles. Thus the knee-joint of the Mylodon must 

 have had one large synovial capsule, that of the Megatherium two, and that of the 

 Megalonyx three such sacs. 



The patella (Plate I. w, and Plate XXIII. fig. 2) is a strong, thick, subtrihedral conical 

 bone, with the base rounded, and also the angle between the two rough outer sides. In 

 the natural position of the bone the base is uppermost, and chiefly composed of a strong 

 tuberosity coarsely and irregularly striated : at the lower half of the outer surface the 

 stria? have a longitudinal and subparallel direction, giving that part of the bone the 

 appearance of an ossified fibrous ligament. On the inner and broadest side the articular 

 surface occupies the upper two-thirds: it is less distinctly divided by a median longi- 

 tudinal rising into two channels than in the Mylodon, being more nearly level and 

 uniform. The non-articular surface below the joint is irregularly grooved and perfo- 

 rated by vascular canals. 



The fabellaj, or post-tibial sesamoid bone (Plate I. «7'), is a smaller subhemispheric 

 bone, with a circular, slightly concave articular surface, which was applied to part of the 

 outer condyle of the femur : the rest of the surface is rough and fibrous, indicative of 

 the imbedding of the bone in a flexor tendon of the leg. 



The tibia and fibula (Plates I. and XXVII. «s, «?) become anchylosed together at both 

 rxtremities in the Megatherium: they are both short, and the tibia presents massive 



• Owes ' On the Mylodon robustus,' 4to. p. 111. pi. 17. 



t lb. 'Fossil Mammalia' of the ' Voyage of the Beagle,' 4to. pi. 25. fig. 5. 



X lb. ' Archetype of the Vertebrate Skeleton,' 8vo. p. 190. 



