374 
in a pointed form along the outer side of the 
facial fork as far as the nasal does on the inner 
side, and an angular fissure is intercepted 
between the anterior extremities of these bones 
into which the pointed posterior part of the inter- 
maxillary bone (fig. 172, 0,0) is inserted. The 
anterior half of the facial fork with its inflected 
end is wholly formed by the intermaxillary 
bones, which thus bound the anterior half of 
the wide external nasal aperture. The small 
detached intermediate bone (fig. 173, a,b) may 
be regarded as a separate centre of ossification 
of the palatine process of the intermaxillaries, 
and of the middle stem which divides the ante- 
rior nostrils in birds and lizards. 
The vomer forms a bony septum, dividing 
the whole extent of the nasal canal from the 
spine of the sphenoid forwards. 
There is a small lachrymal foramen (fig. 
169, 1) at the anterior and inner part of the 
orbit in both the genera of Monotremes; a 
little lower down is the commencement of the 
ant-orbital canal. This canal branches in the 
Echidna, and terminates on the outer side of 
the maxillary bone by a succession of small 
foramina; but in the Ornithorhynchus, where 
it transmits a much larger sensitive nerve, it 
divides into three canals, of which one emerges 
beneath the uncinated process of the maxillary 
above mentioned ; a second descends and opens 
upon the palate ; and the third passes forwards 
into the substance of the facial fork, and termi- 
nates by a large foramen at the outside of the 
intermaxillary bone. 
On the exterior of the cranium the ridges 
indicating the extent of the temporal muscles 
are clearly developed in the Ornithorhynchus, 
and correspond with the stronger zygomata and 
the more complete apparatus for mastication 
in this Monotreme. Four linear impressions 
upon the upper surface of the skull diverge 
from the middle of the lambdoidal ridge, and 
terminate at the temporal ridges. The occipital 
foramen (fig. 173, c) has a vertical plane, as in 
the Echidna, and has a similar rounded notch 
at its upper part. 
The interior of the skull offers many unusual 
modifications. The sella turcica is elongated 
and narrow in both Monotremes; it is bounded 
ge very distinct lateral walls in the Echidna. 
e posterior clinoid processes are chiefly 
remarkable for their height in the Ornitho- 
rhynchus. The semicircular canals stand out 
in high relief in this species, as in Birds. In 
the Echidna the ethmoid encroaches upon the 
anterior part of the cranial cavity in the form 
of a large convex protuberance made by the 
posterior wall of the olfactory cavity, and a 
very extensive cribriform plate is developed. 
In the Ornithorhynchus the olfactory tract is 
comparatively small, in the form of a depres- 
sion, and the nerve escapes by a single foramen 
at the anterior part of the ethmoidal plate. 
This is likewise an interesting mark of affinity 
to the bird and reptile; but the most remark- 
able and characteristic feature in the interior 
of the skull of the Ornithorhynchus is the bony 
falx (fig. 173, 8). This is not present in the 
MONOTREMATA. 
Echidna. The tentorium is membranous in 
both Monotremes. d 
The lower jaw consists in the Echidna of 
two long and slender styliform rami without a 
symphysial joint, but loosely connected together 
at their anterior extremities. An 
cess divides the horizontal from the 
ramus, which rises at an open angle and ter- 
minates in a small oblong convex condyle. A 
short obtuse coronoid process extends from the 
waper part of the horizontal ramus as far in 
advance of the angle as the condyle is behind 
it. The rest of the ramus is rounded likea rib, 
and diminishes to the anterior extremity. _ = h 
dental canal commences below the 
process and divides in its progress, one branch 
terminating near the middle of the smooth 
alveolar border, the other close to the end of the 
ramus. In no Mammiferous animal does .the 
lower jaw bear so small a proportion to the sk 
or to the rest of the skeleton as in the Echidna. 
In the Ornithorhynchus the lower jaw is 
much more developed (fig. 173, £). Each 
ramus commences teriorly by a ae sub- 
hemispherical condyle, the convexity of which, 
so characteristic of the Mammalian class, is 
strongly marked. The ascending ramus is 
nearly horizontal, flattened below, and con- 
tinued upwards in the form of a low vertical 
compressed plate, on each side of which there 
is a deep fossa. The ascending is continued 
by a gentle curve into the horizontal u 
and the angle of the jaw is very feebly indicated. 
The horizontal ramus suddenly expands and 
sends off above in the same transverse line 
two short obtuse processes, both of which 
might be termed ‘ coronoid;’ this structure is 
peculiar to the Ornithorhynchus. The oa 
most process (c), although the largest, is the 
superadded structure, as it affords insertion to 
the internal pterygoid. About two lines anterior 
to these processes the upper border of the 
horizontal ramus expands to form the shallow 
oblong alveolus (e) for the horny grinder. Its 
floor is perforated by several large foramina. 
The dental canal divides; one branch opens by 
a wide elliptical foramen on the outside of the 
ramus immediately anterior to the 
the other terminates at the lower part of the 
end of the ramus. The rami of the jaw con- 
verge and are united at a symphysis of more 
than half an inch in length; there they become 
expanded and flattened, then again disunite, 
and are continued forwards as two 
processes (b), which diverge from each other to 
their broad rounded terminations, and are 
situated just behind the inflected extremities of — 
the similarly separated inter-maxillaries (fig. 
173, a, c,c). On the outer sides of the upper — 
surface of the broad symphysis are the, ae 
and narrow sockets of the two anterior trenchant 
horny teeth. The Monotremes differ from the 
Marsupials in the absence of the inflected pr 
cess developed from the angle of the lower jaw. — 
Of the vertebral column.—Both Monotremes — 
have twenty-six true vertebre, of which the — 
seven first are cervical. In the Echidnasixteen, — 
and in the Ornithorhynchus seventeen, of the a 
ye 
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