OSTEOLOGY.
Of the skull.—The skull in both genera of Monotremata is long and depressed, but is cha racterized by a relatively larger cranium in pro portion to the face than in the Marsupials. The parietes of the expanded cerebral cavity are rounded, and their outer surface is smooth. These characters are most conspicuous in the Echidna, in which the jaws are slender, elon gated, and gradually diminish forwards to an obtuse point, so that the whole skull resembles the half of a pear split lengthwise. The facial angle of the Echidna is 36°, that of the Orni thorhynchus 20°, being almost the lowest in the mammiferous class. The cranial bones and their constituent pieces continue longer dis tinct in the Echidna than in the Ornithorhyn chus ; and their relative position, their con nections, and the proportions in which they enter into the formation of the skull, have been in great measure determined and described in that genus.* I have had the opportunity of investigating the composition of the cranium, a point so im portant in regard to the natural affinities of the Monotremata, in the young Ornithorhynchi transmitted to the Zoological Society of Lon don by Dr. Weatherhead ; and the comparison of this part of their anatomy has enabled me better to appreciate and understand peculiarities of the same part in the Echidna, the skull of which is here also described from original spe cimens.
In the cranium of a young but full-grown specimen of Echidna setosa, (g,fig.169, 170, 171,) the four elements of the occipital bone are unanchylosed and are joined together by smooth linear harmoniT. The basi-occipital (fig. 170, a) presents a six-sided rhomboidal figure, with the posterior margin notched to complete the lower boundary of the large vertical occipital foramen, and thickened and smoothly rounded to form the inferior extremities of the two occipital con dyles. These condyles are principally deve loped from the ex-occipital elements (jigs. 169, 171, b, b), which are expanded superiorly and terminate in an angle wedged in between the supra-occipital and petrous bones ; they extend, but do not meet, above the occipital foramen, being separated by a notch closed by membrane in the recent state. The supra occipital element (figs. 169 St 171, c) is a transversely oblong quadrilateral plate of bone; its short lateral margin is joined by a linear harmonia with the upper part of the os petro sum, on each side ; the wide anterior mar gin is similarly articulated with the single parietal bone, and is slightly overlapped by its posterior margin ; this representative of the deltoid suture runs straight across the posterior and upper part of the skull.
In the specimen in which the preceding condition of the occipital vertebra was mani fested there was no trace of sagittal suture ; the upper and middle region of the cra nium was covered by a single broad, slightly convex, parietal bone, (fig. 169, d,) joined posteriorly, as above described, with the supra occipital, laterally with the petrous and sphe noid bones, and anteriorly with the sphenoid and frontal bones, which the parietal overlaps by a squamous modification of the coronal suture. The part described in this Article as a lamelliform portion of the petrous bone, (fig. 169, e,) which extends upon the lateral and part of the posterior region of the skull, is regarded by the Editors* of the Lecons d'Ana tomie Compar6e, Ed. 1837, as the squamous portion of the temporal; and the flat oblong bone, (fig. 169, B,) which forms part of the la teral wall of the cranial cavity and the posterior half of the zygomatic arch, and which supports the articular surface for the lower jaw, is thought to be the malar bone. But when we consider the low development or total disappearance of the malar bone in the skull of the Insectivore generally, as in Echinops and Centetes among the Fent, and as in the edentate Manic and it is unlikely that the malar bone should attain so superior a size and fulfil such important functions in the Monotrema tous Edentata, in which its condition, according to the above views of the editors of the Lecons d'Anat. Comparee, would be unique in the mammiferous class. It appears to me to be more reasonable to regard the malar bone as either altogether absent in the Echidna, as it is in the Manis, and the zygomatic arch as being completed in the Echidna by a greater exten sion of the zygomatic processes of the tem poral and superior maxillary bones; or else to suppose that these are actually united, at an earlier period, by a separate intervening fugal style, which, however, I have not been more successful in finding than the Continuators of Cuvier. NVith respect to the great develop ment which the petrous bone, according to my view, must present in the Echidna, it may be observed that this bone forms part of the occi pital region of the skull in most Marsupials, and also contributes as large a proportion to the lateral parietes of the skull in certain Ro dents, as the Helamys, as it is here described to do in the Echidna.