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, h). 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. Cuvierl- 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, a, 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 foramen, extends forwards 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, o,o) 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 intermaxillarics, and of the middle stem which divides the ante rior nostrils in birds and lizards.
The vonicr 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. 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 by two very distinct lateral walls in the Echidna. The posterior clinoid processes are chiefly remarkable for their height in the Omitho 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, a). This is not present in the Echidna. The tentorium is membranous in both Monotremes.
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 angular pro cess divides the horizontal from the ascending ramus, which rises at an open angle and ter minates in a small oblong convex condyle. A short obtuse coronoid process extends from the upper 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 like a rib, and diminishes to the anterior extremity. The dental canal commences below the coronoid process and divides in its progress, one branch terminating natr 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 skull or to the rest of the skeleton as in the Echidna.
In the Ornithorhynchus the lower jaw is much more developed (fig. 173, E). Each ramus commences posteriorly by a large 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 mmus, 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 inner 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 rainus 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 alveolus, 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 spatulate 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 long and narrow sockets of the two anterior trenchant horny teeth. The Monotremes differ from the Marsupials in the absence of the inflected pro-. cess developed from the angle of the lower jaw.