Some of the Marsupials, as the Wombat, resemble the Echidna in the open state of the tympanic cavity in the dry skull ; but the most essential points of correspondence with the cranial anatomy of the Echidna are found, as might be expected, in the Ornithorhynchus. In this Monotreme the tympanic cavity (fig. 173, A, k) is a simple excavation at the under part of the petrous bone; the periphery of the opening, which looks almost directly downwards, is en compassed by the tympanic and malleal bones, (fig.173, D, a, b,) the outer and anterior part of the circle being formed by the os tympanicum. The tympanic cavity is relatively smaller, but is defended posteriorly by a larger process sent downwards by the petrous bone near its outer side. The petrous bone here forms no palatal process, but the bony roof of the mouth is ter minated by the pterygoid plates, (fig. 173, A, i,) which meet below the nasal canal, as in the Echidna, but are not divided by any posterior fissure. In a skull of the Ornithorhynchus, in which the suture dividing the palatine processes of the maxillary bones from the bony palate posterior to them remains, there is no trace of a division between the pterygoid and palatine bones, which contribute to complete the osseous palate (fig. 173, A, e).
T/ie occipital bone of the young Ornithorhyn ehus corresponds with that of the Echidna in the relative size and position of its four component parts. The ex-occipitals are shown at fig. 172, b, b, and the supra-occi pital at c. The petrous element of the temporal (e) likewise sends a thin plate to form the poste rior part of the side of the cranium, but it does not intervene between the pa rietal bone and squamous part of the temporal, as in the Echidna. The middle of the upper mar gin of the cranial plate of the petrous bone is notched, and a small vacuity here intervenes between the petrous and parietal bones, which is closed by the squamo-temporal (f), the upper margin of which overlaps the descending pos terior angle of the parietal bone (d). The form of the squamous element of the temporal is very remarkable in the Ornithorhynchus : its ascending cranial or proper squamous portion is a small sub-quadrilateral bony scale, narrower antero-posteriorly, but higher than the corres ponding part in the Echidna ; articulated by its squamous marain with the parietal and petrous, but separated from the latter, at its lower 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 part of the petrous bone. The glenoid plate is equal in size to the squamous process. They meet at a right 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 (fig. 172, i), which is 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 presents two membranous spaces. The skull differs 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 cranial bones are much sooner obliterated, and the whole upper and lateral parietes then con sist of a thin continuous dense plate of bone without diploe.
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 petrous 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 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 upper root of the zygoma can be the analogue of the os quadratum in the Ovipara.