PEOFESSOE OWEN ON THE MEGATHEEIUM. 
813 
From the cast of a distal epiphysis, transmitted by Dr. Harlan to the Museum of 
the Eoyal 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 XXXVIII. fig. 2, and Plate XVII. 66', Philosophical Transactions, 
1855) is a strong, thick, sub trihedral conical bone, with the base rounded, and also the 
angle between the two rough outer sides. In the natm-al 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 siuface the striae have a longitudinal and subparallel 
dhection, 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 longitudinal rising into two channels than in the 
Mylodon, being more nearly level and uniform. The non-articular surface below the 
joint is iiTegulaiiy grooved and perforated by vascular canals. 
Thefabella*, or post-tibial sesamoid bone (Plate XVII. e?'. Philosophical Transactions, 
1855), 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 (Plate XVII. ee, e?, tom. a'^.) become anchylosed together at both 
extremities in the Megatherium : they are both short, and the tibia presents massive 
proportions corresponding with those of the femur. Its proximal end (Plate XXXIX. 
fig. 3) presents two distinct and well-marked articular surfaces ; the inner one (?) is concave, 
the outer one {e) is convex : the extent of these surfaces corresponds with the breadth 
of the articular part of the outer and inner femoral condyles respectively. The back 
part of the outer facet which bends downward affords an articulation ( f) to the fabella. 
The rough interspace between the articular surfaces is a little concave transversely, and 
convex from before backward ; its breadth equals that of the outer surface : it developes 
no intercondyloid process for crucial ligaments. 
The fore part of the proximal end of the tibia presents a large irregularly triangular 
rugged protuberance for the ligamentum patellae ; the back part, below the outer con- 
dyle, developes a smaller but more prominent rugged process, the apex of which over- 
hangs the upper part of the interosseous space : between this ‘ post-tibial ’ process and 
the rough inner border of the bone there is a deep and wide longitudinal channel 
inclining a little obliquely to the interosseous space. The shaft of the tibia gradually 
contracts to its middle, and as gradually expands to its distal end. It is subcompressed 
from before backward ; is smoother behind than in front : there is a longitudinal 
channel on each side the back part of the lower end of the tibia, which forms a con- 
vexity between them. 
* Owen, ‘Archetype of the Vertebrate Skeleton,’ 8vo, p. 190. 
