MONOTREMATA. 
rower corresponding bone in the Echidna. 
These bones, which are called the ‘ epicoracoids’ 
(fig- 173, a,nn) are joined by their median 
margin to the stem of the episternal, and by 
their anterior margins to its transverse branches, 
which are overlapped by the epicoracoids. 
e true or acromial clavicle (fig. 173, 4, 
mm) is a long, slender, compressed, slightly 
bent bone, continued from the articular cavity 
at the end of the acromion to near the median 
line of the episternum, anterior to but parallel 
and in contact with the branches of the epi- 
sternum, with which the clavicles finally co- 
alesce, but at an earlier period in the Orni- 
thorhynchus than in the Echidna. These 
clavicles are the homologues of the os fur- 
catorium in the bird: the T-shaped episternum 
(fig. 173, a, t) is feebly represented in birds by 
e€ median process continued forwards be- 
tween the coracoid articular cavities from the 
fore part of the sternum. It is in Lizards, and 
especially in the extinct Ichthyosaur, that the 
episternum presents the same form, develop- 
ment, and relation to the clavicles, as in the 
Monotremes. The epicoracoids again are want- 
ing in the bird, but they are present in lizards, 
and the remarkable breadth of the coracoid in 
the Enaliosauria is due to their presence, al- 
though, singularly enough, they are anchylosed 
to the coracoids in these extinct reptiles, while 
in the warm-blooded Monotremes they remain 
separate. In the Echidna they are articulated 
with the coracoid bya true synovial joint. To 
Tender the resemblance between the Mono- 
‘treme and the Bird complete, in respect of the 
structure of the scapular arch, the episternum 
‘must be reduced to a short and simple process 
attached to the anterior part of the manubrium 
Sterni, the epicoracoids must be removed, and 
the clavicles anchylosed together at their mesial 
extremities.* 
_ The humerus is a short and strong bone, 
expanded at both extremities, and, as it were, 
twisted half round upon itself. The proximal 
expansion terminates by a broad thick convex 
border, the middle part of which is developed 
into the articular head, which is so adapted to 
the glenoid cavity, that the bone is maintained 
in a horizontal position, and the distal expan- 
sion is nearly vertical. The deltoid and pec- 
toral crests are strongly developed ; both con- 
dyles are remarkably produced, especially the 
internal one, which is perforated. ( Fig. 173, 
H, a.) The distal articular surface scarcely 
occupies a fourth part of that broad termination 
of the humerus: it presents, in the Echidna 
(fig. 168), a convex tubercle, which is broadest 
in front, for the articulation of the radius, nar- 
row behind, for that of the ulna. The articular 
surfaces of both anti-brachial bones are concave : 
so that the elbow-joint admits freely of flexion 
and extension, abduction and adduction, but is 
restricted in the movement of rotation. 
* For a full and elaborate discussion of the 
various opinions which have been offered respect: 
ing the homology or signification of the complicated 
apparatus of the shoulder in the Ornithorhynchus, 
the reader is referred to Meckel, ¢ De Orsithorhyn: 
cho,’ &c. pp. 12-15. 
377 
The radius and ulna are in contact and 
pretty firmly connected together through nearly 
their whole extent; the interosseous space 
being reduced to a slight fissure. The una is 
chiefly remarkable for the olecranon, (fig. 173, 
4, u,) which is bent forwards upon the humerus, 
and transversely expanded at its extremity, 
especially in the Ornithorhynchus, in which the 
lower or inner angle of the expanded ex- 
tremity is considerably produced. The shaft 
of the ulna is compressed, and increases in 
breadth, in the Echidna, as it approaches the 
broad carpus. In the Ornithorhynchus it is 
bent like the italic /, is more cylindrical, and 
more suddenly expanded at the distal end. 
The radius offers little worthy of notice, except 
that in the Ornithorhynchus it is flattened next 
the ulna, and so applied to that bone as to 
prevent altogether a rotation of the hand upon 
the ulna. In the Echidna the distal articular 
surface of the ulna (fig. 175, n) presents two 
convex trochlee separated by a median con- 
cavity; that of the radius (fig. 175, r) offers 
a reverse condition: here two concavities are 
divided by a median convex ridge ; all the four 
facets at the carpal joint of the antibrachium 
are in the same transverse line. The two 
radial concavities receive the two articular 
convexities of the broad scapho-lunar bone 
(fig. 176, a): the two: convex trochlee of the 
ulna play upon two concavities, one-half of 
each of which is contributed by the cuneiform 
(fig. 176, 6) and pisiform bones (c). This com- 
plicated joint limits the movement of the hand 
upon the fore-arm to flexion and extension. 
Fig. 175. 
Bones of fore-foot, palmar aspect, Echidna setosa. 
( Original. ) 
Notwithstanding the confluence of the sca- 
phoid with the lunar bone in the carpus of the 
Echidna, as in that of the Marsupials and 
Carnivora, it includes eight ossicles, a small sesa- 
moid bone (fig. 176,* ) being developed in the 
tendon of the flexor carpi radialis, and articu- 
lated with the scapho-lunar bone and radius. 
The distal series of the carpus includes the four 
normal bones, the trapezium. (fig. 176, h) 
