PBOPESSOE GWEN ON- THE MEGATHEBITJM. 
811 
when engaged in -supplying their daily wants; whence it may be inferred that the 
exertion of such forces was associated with equally peculiar habits in the megatherioid 
animals. 
The femur of the Megatherium (Plate XXXVIII. fig. 1) is one of the most massive 
limb-bones in the Mammalian class; from its proportions it might rank with the ‘flat’ 
instead of the long bones, but that its thickness would rather bring it into the categoiy 
in which the caipal or tarsal bones are placed according to the old anatomical character 
derived from shape. 
The head (Plate XXXIX. fig. 1) would be a smooth hemisphere, but that the antero- 
posterior diameter somewhat exceeds the transverse one. Its surface is unimpressed, 
and its periphery uninterrupted, save by a small entering notch at the middle of its back 
part, into which possibly some ligament like the ‘ ligamentum teres ’ may have heen 
implanted. The neck of the femur is short and ill-defined ; the upper contour passes 
from the head to the summit of the great trochanter (Plate XXXVIII. t), which is on 
the same level, m a slightly concave line; the inner contour of the bone descends with 
a somewhat deeper concavity from the lower periphery of the head to the shaft. This 
is flattened fr’om before backward, and presents a slightly oblique twist, the head and 
the outer condyle being on a plane anterior to the trochanter and inner condyle. The 
gi-eat trochanter presents a broad rugged surface, flattened obliquely from above 
downward and inward, divided into two somewhat flattened facets by a transverse ridge 
arching upward. The lower facet contracts as it descends, and is continued into the 
strong outer ridge which descends to the external condyle. The thick rough border 
of the rest of the trochanter stands out beyond the contiguous parts of the femur ; least 
so at the upper, and most at the back part of the process, where the border defines 
outwardly a small but deep trochanterian fossa (ib. /). This fossa is bounded below by 
a small tuberosity. The small trochanter is represented by a rough ridge, 6 inches 
long, 2 inches broad, occupying the middle third of the shaft a little anterior to its inner 
border. A few shorter longitudinal ridges occur on the fore part of the shaft between 
the upper end of the small, and the upper and fore part of the great trochanter. The 
rest of the anterior surface of the femur is smooth, concave lengthwise, slightly convex 
transversely. 
The back part of the femur presents a small low tuberosity below its middle part, 
near the inner border ; the triangular siu’face between this tuberosity, the head, and the 
gi’eat trochanter, is smooth and flat. The contour of the back part of the femur, from 
the head to the outer condyle, is convex; that from the trochanter to the inner condyle 
is concave ; the lower half of the back part of the shaft is convex transversely. The 
lower end of the femur (Plate XXXIX. fig. 2) presents two articular surfaces, the inner 
one (ib. ^) being that of the internal condyle, the outer one being the combined ectocon- 
dyloid {e) and rotular (r) surfaces. The latter is extensive, and describes a semicircle 
fr’om before backward, but is naiTow from side to side ; in this direction the rotular portion 
is slightly concave; its limits are indicated by a notch on the inner side: the condyloid 
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