following vertebre support long and functional 
ribs, so that there are three lumbar vertebre in 
the Echidna, and but two in the Ornithorhyn- 
chus, which thus resembles the Lizards in the 
great proportion of the trunk which is encom- 
d by the costal arches. Another approxi- 
mation to the Oviparous type is made by the 
long-continued separate state of the short 
cervical ribs in both Monotremes: these in a 
young but nearly full-grown Echidna are 
detached from all the cervical vertebra except 
the atlas. The vertebral end of the cervical 
_ rib is bifurcated; the lower branch, represent- 
ing the head, is articulated to the transverse 
_ process or tubercle developed from the body 
_ of the vertebra; the upper branch, represent- 
_ ing the costal tubercle, is articulated to a trans- 
‘verse process developed from the side of the 
age the neural arch. In the Ornitho- 
vhynchus the cervical ribs appear to become 
earlier anchylosed to the vertebra, except as 
_vegards the axis, in which the broad costal 
appendage retains its original independence 
. throughout life, and is slightly moveable 
upon the confluent extremities of the two 
‘transverse processes. In the succeeding ver- 
_tebre the space intercepted between the two 
_ transverse processes, the vertebra, and the cos- 
tal rib, forms the so-called ‘ perforation of the 
transverse process’ for the vertebral artery in 
_humananatomy. In the Echidna, above alluded 
‘to, the ‘ neurapophyses’ or vertebral plates of 
the atlas, which together form the neural or 
_ Spinal arch, were unanchylosed at their upper 
or spinal extremities. The atlas of the Orni- 
i trrnchus is chiefly distinguished from that 
of the Echidna by the continuation, from the 
Tower part of its slender body, of two long 
_ diverging processes which are developed in the 
_ strong tendons of the recti capitis antici 
_ muscles. The spine of the dentata is broad 
and high in both Monotremes; those of the 
other cervicals progressively diminish in size 
‘in the Ornithorhynchus, but become at once 
néarly obsolete in the Echidna. The transverse 
_ and spinous processes are of moderate size in 
_ the rest of the true vertebre, but are largest in 
the lumbar region. The posterior oblique 
‘processes are single in these vertebre. The 
articular surfaces of the vertebre, which are 
_ slightly concave, are joined together by a thick 
_ ¢ireular band of ligamentous fibres (fig. 174, a) 
_ attached to the circum- 
ference of the articular 
Surface, enclosing a cen- 
_ tral oblate spheroidal ca- , 
_ vity ) lined bya synovial “eee 
_ membrane and filled with 
fluid. 
The ribs are long and 
Slender in the Ornitho- 
_ thynchus, somewhat stron- 
es cee the Echidna. The 
a is flattened, the rest 
are cylindrical. Each rib 
is articulated by a single 
joint, uniting the head to 
the. vertebral. interspace, Intervertebral cavities, 
or to the side of the cen- Echidna. ( Original. ) 
Fig. 174. 
a} 
+ 
ee 
MONOTREMATA. 
375 
trum, as in the two last pairs: the tubercle, 
though small, is distinctly developed, and de- 
fines the neck of the rib, although it does not 
join the transverse process of the vertebra. 
Meckel’s statement, ‘ tuberculum adest nul- 
lum,’ is applicable only to the last three or 
four pairs of ribs. The first six pairs are joined 
to the sternum in both the Ornithorhynchus and 
the Echidna. Six pairs of ossified sternal ribs 
(hemapophyses*) are articulated to the ster- 
num in the Echidna; the first four are nearly 
straight and sub-cylindrical; the fifth and sixth 
are expanded. Five pairs of ossified sternal 
ribs are present in the Ornithorhynchus, to 
which the second to the sixth vertebral ribs 
inclusive are joined by shorter intervening 
cartilaginous pieces of a similar form. The 
first rib in the Ornithorhynchus is joined to the 
sternum by cartilage alone. The interposed 
cartilages, which thus form a third element in 
the costal arch, repeat a structure common in 
crocodiles, and may be regarded as the homo- 
logues of the costal appendages in the ribs of 
birds, which in this class are removed from the 
interspace of the vertebral and sternal rib, and 
articulated to the vertebral piece. 
The cartilages of the false ribs in both genera 
are singularly expanded and flattened, and 
present an imbricated arrangement, gliding 
upon each other with a slight yielding motion : 
the last vertebral rib, which is the shortest and 
straightest, is terminated by a short free carti- 
lage. Many bone-corpuscles are scattered 
through these cartilages. 
The ordinary sternum, to which the true ribs 
articulate, consists of four ossicles, and in the 
Echidna a fifth is developed in the base of the 
ensiform cartilage; the most anterior of these 
ossicles (fig. 173, a, s) has the usual expanded 
hexagonal form and large proportions of the 
manubrium sterni, and receives, besides the 
first and part of the second pair of ribs, the 
extremities also of the coracoid bones. The 
ensiform cartilage in the Echidna presents an 
elongated oval form with a central perforation. 
A large T-shaped episternal bone (fig. 173, , t) 
is articulated to the anterior surface of the 
manubrium sterni; it is the key-bone of the 
complicated scapular arch. 
The sacrum consists in the Ornithorhynchus, 
as in most Saurians, of two vertebra, distin- 
guished by the greater breadth and thickness 
of their transverse costal processes. In the 
Echidna there are three sacral vertebra. 
There are thirteen caudal vertebre in the 
Echidna (fig. 168). The first is the largest, 
with broad transverse processes, the rest pro- 
gressively diminishing, and reduced, in the six 
last, to the central element. The Ornithorhyn- 
chus (fig. 173, a) has twenty-one caudal ver- 
tebre, of which all but the last two have 
* In the complete vertebre which encompass 
the centre of the vascular system, the hemapo- 
physes or halves of the chevron-bone, or inferior 
vertebral laminz, are retained, together with the 
inferior or hemal spine. The series, sometimes 
anchylosed, as in Man, of these spines forms the 
* sternum :’ the inferior lamine or hemapophyses 
are the sternal ribs. wie 
