MONOTREMATA. 
trous, but separated from the latter, at its 
ower half, by a fissure analogous to but wider 
than the canal which traverses the squamous 
union of the same bones in the Echidna. The 
base of the squamous plate is contracted but 
thickened to form the origin of the zygomatic 
process : it sends inwards a thin plate, concave 
externally, which forms the glenoid cavity for 
the lower jaw, and is applied against the basal 
of the petrous bone. The glenoid plate 
is equal in size to the squamous process. They 
meet at aright angle, and, from the tubercle 
developed from their union, external to the 
glenoid cavity, the zygomatic process is con- 
tinued forwards to join that which is sent off 
by the superior maxillary bone. I could not 
find any distinct malar bone in the young 
Ornithorhynchus. The same arguments against 
considering the squamous bone to be the malar 
apply to this Monotreme as have been used in 
‘reference to the Echidna. 
The parietal bone (fig. 172, d) forms the 
chief part of the upper region of the skull; it 
is longer in proportion to its breadth than in 
the Echidna. Both the lambdoidal and co- 
ronal sutures are squamous, and the parietal 
overlaps the frontal bones. There is no trace 
of sagittal suture; the bony falx is developed 
from the line which that suture would occupy. 
The lateral connexions of the parietal differ 
from those in the Echidna by its union with the 
Squamous portion of the temporal bone, as 
above-mentioned. 
The frontal bones (h,h) are relatively smaller 
than even in the Echidna: they were divided 
by a suture in the specimen described. The 
form of their exposed surface is shown in 
fig. 172, h, h. 
The side of the skull anterior to the petrous 
bone is formed by the great ala of the sphenoid 
(he. 172, i), whichis joined by a well-defined 
linear harmonia to the parietal bone. 
The Ornithorhynchus differs from the Echidna 
- in the large vacuities in the floor of the skull 
behind and in front of the tympanic cavity, the 
one representing the combined jugular and 
condyloid foramen, the other the oval foramen, 
between which the body of the sphenoid also 
nts two membranous spaces. The skull 
iffers also in the larger size of the foramen 
rotundum, in the stronger zygomata, the more 
complete orbit, and the singular modification 
__ of the bones supporting the beak. The cranial 
cavity is proportionally smaller and shallower 
in the Ornithorhynchus. The sutures of the 
_ €ranial bones are much sooner obliterated, and 
the whole upper and lateral parietes then con- 
Sist of a thin continuous dense plate of bone 
without diploé. 
The resemblance which the Ornithorhynchus 
offers in this respect to the class of Birds is 
noticed by Meckel. 
_ The oblique canal, (fig. 173, c, c,) which 
traverses the squamous suture between the 
and squamous portions of the temporal 
in the Echidna, is so much shorter and wider 
in the Ornithorhynchus that it appears to de- 
tach from the side of the cranium a distinct 
373 
superior column or root (fig. 173, c, a) to the 
posterior commencement of the zygomatic arch. 
An analogous canal runs between the tym- 
panic and mastoid bones in the skull of the 
Crocodile, and is dilated to great width in the 
Lizards ; but the presence of a distinct tym- 
panic bone in the usual position in the Orni- 
thorhynchus nullifies the supposition that the 
ie root of the zygoma can be the analogue 
of the os quadratum in the Ovipara. 
The articular surface for the lower jaw 
(fig.173,c, b) is much more distinctly developed 
than in the Echidna; it occupies the base of 
the zygoma, is extended and concave trans- 
versely, narrower and slightly convex from 
before backwards: it is not defended by any 
posterior process. The zygoma is complete, 
and consists of a nearly vertical straight plate 
of bone, expanded at its anterior or maxillary 
extremity, where it sends upwards an angular 
process bounding the orbit posteriorly, and 
bends downwards to support the broad alve- 
olus of the horny molar tooth (fig.173, a, g, A). 
The orbits are small and directed obliquely 
upwards and outwards. The skull is more 
contracted between them than in the Echidna, 
but anterior to them it begins to expand, be- 
comes flattened horizontally, then bifurcates, 
and the two depressed branches, after slightly 
diverging, terminate each by an inwardly 
inflected process, the extremities of which 
are half an inch apart. The space inter- 
cepted by the facial fork is the external bony 
nostril (fig. 172, p), which is thus left incom- 
plete anteriorly. The forms and proportions 
in which the bones of the face enter into the 
formation of the external nasal aperture are 
illustrated in the figure of the cranium of 
the Ornithorhynchus given by Pander and 
D’Alton,* and I have verified the accuracy 
with which the sutures are there delineated. 
Cuviert supposed the facial forks to be formed 
by the intermaxillary bones, and describes a 
small bone in the middle of their interspace 
(fig. 172, p.) suspended in the cartilage of the 
upper mandible, and with an emargination on 
each side of its inferior plate, which he conjec- 
tured might represent the nasal bones and the 
palatine part of the intermaxillary bones. The 
true nasal bones (fig.172, n,n) are, however, 
as shown by Pander and D’Alton, analogous in 
situation to and more largely developed than 
those of the Echidna. They commence each by 
an angular process, which overlaps the frontal, 
and extends into the inter-orbital space. They 
are continued forward of equal breadth, and 
have their anterior extremity obliquely trun- 
cated and terminated in a fine point, which 
extends to the middle of the inner side of the 
facial fork. The nasal bones thus form the pos- 
terior half of the boundary of the bony nostril. 
The superior maxillary bone (fig. 172, m, m), 
after sending off a process (c), which curves 
over the ant-orbital at extends forwards 
* Skelete der zahnlosen Thiere, 1825, (tab. ii. 
fig. a.) . 
+ Legons d’Anat, Comp. 1837, ii. p. 459. 
