The side of the cranium is principally formed by the largely developed petrous bone (fig.170, e) and the great ala of the sphenoid, which meet and are joined in the interspace separating the parietal from the squamo-temporal bone, by a nearly vertical harinonia half an inch long ; this harmonia is crossed at nearly right angles by a deep groove, which in some parts sinks through the wall of the cranium and exposes its cavity. The groove or canal first runs be tween the squamous and petrous elements of the temporal bone, and is a conspicuous fea ture in the skull of the Ornithorhynchus.
The lower part of the side of the skull of the Echidna is closed by the squamous element of the temporal, which overlaps a large portion of the petrous hone, and by a small portion of the sphenoid : it is represented detached from the skull at fig. 169, B. The lower boundary of the squamo-temporal forms a straight line, from which the glenoid surface for the lower jaw (f) is extended inwards at a right angle, upon the base of the skull ; the anterior part is continued forwards, protecting the temporal fossa by a thin vertical plate of bone, and then diminishes to a slender, straight, styliform, zygomatic process which rests obliquely on a corresponding process of the superior maxil lary bone.
The tympanic cavity is shallow, and exca vated in the basal part of the petrous bone, where it is widely open in the macerated skull : it is figured closed by the tympanic bone and membrane at g, fig. 170, and exposed by their removal at e" fig. 170. The plane of the mem brana tympani is horizontal, and its exter nal surface looks nearly downwards. Three fourths of its circumference are implanted in the groove of the very slender incomplete hoop formed by the detached tympanic bone, which is figured with the anchylosed malleus at c,fig. 169. The petrous bone is continued from the tympanic fossa forwards and inwards, in the form of a broad and nearly horizontal process, (fig. 170, to the pterygoid plate of the sphe noid, (i, i',) which is also horizontal. The pe trous and pterygoid plates are joined by an oblique harmonia, and both contribute to extend the bony palate backwards. The palatal pro cess of the petrous bone is abruptly terminated behind by the Eustachian groove (fig. 170*).
The frontal bone (fig. 169,h) in the cranium here described was divided by a median frontal suture, toothless like the rest ; the angle be tween the superior and the orbital plates is rounded off; the orbital plate joins the vest ala of the sphenoid by a deeply sinuous suture. The anterior part of the frontal is largely over lapped by the bases of the nasal bones, which encroach upon the interorbital space.
The nasal bones (fig. 169, n) receive the upper edge of the superior maxillary bone into a groove at their outer margin, and articulate anteriorly with the intermaxillaries (fig. 169, a); but these meet above the nasal canal in front of the nasal hones for an extent of about three lines, and thus exclusively form the boundary of the single, oval, and terminal external nostril. The lower or palatal process of the interinaxillary extends backwards in the form of a long and slender pointed process which is wedged into a fissure of the superior maxillary bone.
The anterior palatal or incisive foramen is a single large elongated fissure extending from the narrow anterior symphysis of the inter maxillaries backwards, for some distance, be tween the palatal processes of the maxillaries. At the back part of the bony palate a narrow fissure extends forwards between the pterygoid bones, and the intermediate extent of the bony palate is entire, or presents only a few minute perforations. The palatal bones, if
originally distinct, soon become confluent with the maxillaries. There was no separate osseous style representing the malar bone between the zygomatic processes of the maxillary and tem poral bones in the skull here described, The zygomatic process of the superior max Wary bone (fig. 169, ns) extends backwards as flu as the posterior boundary of the zygomatic or temporal fossa ; the palatal process extends along the floor of the orbit in a similar form and to nearly the same extent. The orbit is marked off from the temporal fossa by merely a slight ridge extending down and across the suture joining the frontal and sphenoid bones.
The skull of the Echidna differs from that of the edentulous Manis and Myrmecophaga in the completion of the zygomatic arches, in the unclosed state of the tympanic cavity, in the large size of the foramen incisivum, and the surrounding of the external nostrils by the intermaxillary bones alone : it differs also in the smaller relative distance between the poste rior palatal fissure and the superior maxillary bones, and in the apparent absence of the pala tine bones, the presence and interposition of which between the pterygoid and maxillary palatal plates elongates the palate in the pla cental Anteaters at the part where it is rela tively shorter in the Echidna. In the modi fication of the pterygoid plates of the sphenoid to complete the posterior nasal canal, the Echidna manifests an interesting resemblance with the great Anteater ; but it differs from this, as from every other mammiferous species, in the palatal plates contributed by the petrous bones to the broad posterior part of the roof of the mouth which supports the horny palatal teeth. Cuvier describes the posterior palatal fissure as extending between the palatine bones, and therefore regards theplates, which are here affirmed to be developed from the petrous bone, as being the pterygoid processes of the sphenoid; and, according to this view, he truly observes that their horizontal position is very remark able;'' but he might have added, that their share in the formation of the tympanic cavity was not less so. The same determination of the bones composing the posterior part of the osseous palate of the Echidna is repeated in the Lev:ins d'Anatomie Comparee, 1837, p. 454. If, however, the sphenoid be sepa rated from the occipital bone, which was easily done in the young skull of the Echidna repre sented infigs.169 and 170, the horizontal plates, described by Cuvier as pterygoids, are left behind, not as separate bones, but as conti nuous portions of the petrous elements of the temporal, which form, at the same time, part of the base of the cranial cavity, complete the inner wall of the tympanum, and the anterior part of the Eustachian groove. The palatines of Cuvier are developed from the sides of the basi-sphenoid, and almost immediately bend inwards and meet below the nasal canal, which they thus prolong posteriorly, as in the Myr inecophaga ; and they are separated poste riorly, also, as in that genus, by an acute fissure, presenting unequivocally the same modifications which characterize the pterygoids in the pla cental Anteaters, and in the Crocodile. The suture dividing the pterygoids from the pala tines in the Echidna is obliterated, if it ever existed ; or the true palatines may be confluent with the palatine processes of the maxillary bones.