FEMUR

The femur

of

Meniscotherium

is comparatively robust, asnoted by cope in his very brief description,

and

its shaft is straight, although proximally the anteromedial

margin

is decidedly curved as it

approaches the head (see pi. 8).

The head may

be

somewhat

oval

to nearly circular in proximal

view and

its neck moderately slender

and

anteromedially directed.

The

fossa for the

ligmentum

teres is noticeably posterior in position

and

essentially marginal, in keeping with the posteriorly directed cotyloid notch of the acetabulum.

The

greater trochanteris robust

and

extends proximallya little

more

than the head,

from which

it is separated

by

a well-defined notch.

The

posterior crest of the greater trochanter partially covers posteriorly a deep

and

elongate digital fossa into

which

are inserted the obturator muscles. Distally this comparatively thick or well-rounded crest appears to divide,

and

a

more

feeble,

sometimes

poorly defined crest or intertrochanteric line crosses the shaft distomedially to join the distal extremity of the medially outstanding, nearly triangular lesser trochanter for theiliopsoas

on

the posteromedial

margin

of theshaft.

The apex

of the lesser trochanter has a position about a quarter of the length of the

bone

distal to the proximal extremity.

The

better defined lateral portion of the posterior crest

from

the greater trochanter extends distally to join the

prominent and

flattened third trochanter,

which

is almost

halfway down

the shaft.

The

develop-

ment

of this process emphasizes, as in the horse, the importance of the superficial gluteus muscle in abducting the limb

and

flexing the hip joint.

The

outline of the third trochanter varies

somewhat from

a proximodistally elongate flare to a shorter but

more

laterally pro- jectingprominence.

On

the enlarged distal extremity the patellar trochlea is narrow, elongate,

and

prominently raised.

The

condyles are large

and

sepa- rated

by

a broadly

open

intercondyloid notch,

and

the lateral

and

medial surfaces of the extremity are strongly divergent posteriorly.

The

condylar tuberositiesorepicondyles are noticeable,

and

the outer,

more

subdued, is at the extremity of the

prominent

lateral supra- condyloid crest

which

extends distally

from

the third trochanter.

A

similar medial supracondyloid crest disappears a short distance proxi- mally

on

theshaft.

Much

that

Cope

said (1884b, pp. 455-456) about the

Phenacodus femur

applies to that of Meniscotherium, particularly with regard to the proximal extremity

and much

of the shaft, but I note that the greater tuberosity in Phenacodus, although anteroposteriorly deeper,

is a little less extended proximally,

and

the shaft is relatively

more

robust. Distally the fossae at the posterior base of the condyles are

much

less significantin

M

eniscotherium,

and

thetransversely oriented posterior portion of the inner articular surface is as

wide

as or wider, rather than narrower, than the

more

oblique or transversely

NO. 2

A STUDY OF MENISCOTHERIUM — GAZIN

6^

more convex

outer articular surface in

M

eniscotherium.

The

patellar trochlea in

Phenacodus

seemsbroad,

and

thepatellaitself is

compara-

tively

wide and

not so elongate

and

distally tapering as in

Menisco-

therium (seepi.8)

.

The

straight or but slightly curved

Hyopsodus femur

is slender, elongate,

and somewhat

flattened anteroposteriorly.

The

proximal extremity is transversely narrow, as the

head and

slender

neck do

not extend medially so noticeably as in

M

eniscotherium.

The

head, moreover, is nearly spherical

and

the fossa for the

ligmentum

teres is centrally located, well

removed from

the

margin

of the articular surface.

The

greater tuberosity is comparatively small

and

does not project proximally as far as the head.

The

^digital fossa is

proximo-

distally

much

shorter than in

M

eniscotherium,

and

the lesser tuber- osity is nearer the proximal extremity.

The

lateral

margin

of the shaft

from

thegreater tuberosity tothelateral condyleis compressed,

and

just

beyond

tothemidpoint ofthe shaftis laterally

expanded

Into awell-defined,proximodistallyelongate third trochanter,witha

rugged

crest for the superficial gluteus muscle.

The

distal extremity of the

Hyopsodus femur

at

hand

is poorly preserved, but I note that the patellartrochleaisrelativelybroader thanin

M

eniscotherium.

TIBIA

The M

eniscotherium tibia is a little shorter than the

humerus.

It is relatively slender

and

noticeably

bowed

^anteriorly (see pi. 9).

It exhibits

an

elongate cnemial crest that is laterally deflected

and

extends straight nearly half the length of the shaft.

The

crest is distally

somewhat

roughened, evidently for the tendon of the semi- tendinous muscle

which would have

its originatthe

prominent

ischial tuberosity.

From

^{this point} the crest rapidly subsides as a well-

rounded

ridge or

margin

crossing obliquely to the medial side

and

then directed

more

or less

subdued toward

the inner malleolus, Proximallythe shaft

shows

abroad, gently

convex

anteromedial sur- face

and

narrower, concave lateral

and

posterior surfaces

which

are separated

by

a sharp posteromedial crest that

would

support the interosseous

membrane.

This crest curves strongly

outward

proxi- mally to

where

it reaches the facet for the

head

of the fibula

and

distally swings

somewhat forward and subdued

to a

more

medial position at the place of contact for the distal extremityof the fibula.

The

posteromedial

margin

of the shaft is also acute proximally, beneath the medial tuberosity or condyle of the tibia but loses this

character distally, near the midpoint of the shaft. Distally, the shaft

is slender

and

essentiallyrounded.

The

proximal extremity of the tibia is broad

and

laterally over- hanging.

The two

surfaces for the

femur

appear roughly equal.

The

medial surface is

somewhat

concave transversely but distally offset

and

broad posteriorly.

Along

the posterolateral

margin

of this there is a noticeable facet, offset or obliquely turned

from

the foregoing

and

adjacent to the popliteal notch, for attachment of the posterior crucial ligament.

The

outer surface for the

femur

rides smoothly

up

the

median

spine v^hich

forms

the medial

apex

of the surface

and

accords with the transversely

more

oblique appearance of the outer articular condyle of the femur.

The

distal surface of the laterally overhanging outer condyle

shows

a small, convex, oval

and

almost distally facing facet for the fibula.

On

^{the anterior}

portion of theproximal extremity there is adistinct transversenotch or step

which

tends to define the proximal extremity of the cnemial crest

and

is evidently the position for attachment of the patellar ligament.

The

distal extremity of the tibia is

somewhat

enlarged

and

exhibits a

pronounced

distal extension medially of the internal malleolus

which

articulates with the inner side of the astragalus.

The

distal articular surface

shows

a shallow, anteroposteriorly elongate con- cavityadjacentto the innermalleolus for the innercrestof the astrag- alus.

The

transversely

more

elongate outer surface for the medial side of the outer crest of the astragalus is decidedly oblique to the longitudinal axis of theshaft. Thisoblique facetterminates

proximo-

laterally adjacenttothefibular contact.

Cope

(1884b, pp. 503-504) has called attention to several dift'er-

ences

between

the tibia of

Meniscotherium and

that of

Phenacodus.

In the character of the cnemial crest, he noted that it is relatively shorter

and more

obliquely truncated proximally,

and

its distal exten- sion does not cross to the internal malleolus in Phenacodus.

With

regard to "the early disappearance of the external posterior angle

and

its reappearance

on

the distal two-fifths of the length of the shaft" in Meniscotherium, I suspect that his specimen

was

unusual or defectively preserved, as

seems

evident in his illustration (1884b,

pi. 25g, figs. 16, a, b,

and

c).

His

fifth point evidently refers to the disappearance

on

the middle of the shaft of the posterior inner angle, rather than the posterior external angle, because as actually stated this

was

part of his fourthpoint.

He

furthernoted that the posterior face of the shaft distally is

roughened

for muscular insertion in

NO. ²

A STUDY OF MENISCOTHERIUM — GAZIN 69

Meniscotherium,

and

that the internal malleolus is obliquely truncate

and

acuminate as in

many

creodonts. In

Phenacodus

the internal malleolus ^is

much more

blunt. It

may

^be further noted that the tibia in

Phenacodus

is essentially straight, not

bowed

as in

Menisco-

therium,

and

the shaftis

much more

robust.

A

distal portion of a tibia is

known

for

Hyopsodus

paulus, as well as for

H.

zvalcottianus.

Both show

the anteroposteriorlyconcave surface for the astragalus

which

lacks

any

clearly defined

median

flexure, denoting a very shallow trochlear groove in the latter. This surface, moreover, is not so oblique transversely as in

Menisco-

therium.

The

internal malleolus is abruptly truncated distally

and

the

rounded

anteromedial

margin becomes more

anterior in position proximally

on

the shaft, in the direction of the cnemial crest.

A

lateralcrest forthe interosseous

membrane

^is

somewhat

betterdefined inthe

H.

paulus specimen, butat^itsdistal extremity, at the

margin

of the astragalar facet, the internal malleolus is roughly acuminate

and

Ifind

no

evidenceofafacetfor thefibula.