Mammalia. —In all mammalia, except man, the articular surface on the squamosal bone is bounded posteriorly, or, in the rodents, inter nally, by a prominent ridge or process. This is the case in the higher Quadrumana, and there is an indication of it even in the lower races of mankind. In the Carnivora the con dyle is in general extremely long, cylindroid, and its length is set, almost or quite directly from side to side, whilst the surface to which it is opposed is bounded by a very salient ridge fore and aft. This condition attains its acme in the Badger, where the salient ridges, especially the posterior, even arch over and surround the cylindroid condyle so much, that in the dry skull the lower jaw remains at tached to the cranium without any artificial contrivance, and can be removed only by slipping each half out sideways when the sym physis has been disjoined. With such a joint there cannot, of course, be the usual lateral motion of the jaw. In the Rodcntia, on the contrary, the long diameter of the con dyle is directed antero-posteriorly, and is ad. opted to an antero-posterior groove, mostly bounded, internally and externally, by salient ridges. Hence that nibbling, antero-posterior motion of the jaw, which is so conspicuous in these animals, and with which their teeth and masticatory muscles are in such admirable relation. This only holds good in respect of the placental rodents. In that great marsupial rodent, the Wombat, the saliently arched, cylindroid condyles have the usual side to si le and converging-behind set of their long diameters. The articular surface which is opposed to this condyle is placed, as in the placental rodents, on the zygomatic process, which in both runs almost directly outwards; instead, however, of being an antero-posterior groove it is a transverse convex cylindroid, describing a retreating curve ; so that in the temporo-maxillary joint of the wombat a cylin droid ridge is opposed to another cylindroid ridge, the one describing a salient, the other a retreating arch.* The articular facets of the temporo-maxillary joint in the Runzinantia approach more nearly to plane surfaces than in any other animals. The squamosal facet is bounded by a ridge posteriorly, but has no bony limitation in front. This is in re lation with the extensive lateral movement of the jaw which these animals use in chew ing the cud, and with the limited power of gaping which they possess. The rest of the mammalia present varieties in this joint which are extremely interesting, but an account of them would occupy more space than the limits of this work permit.
Ayes. — In birds, as above stated, the arti. cular surface of the lower mandible is con cave, and is adapted to the tympanic bone, or, as the older ornithologists called it, the os quadratum.f The articular surface of the lower jaw presents two concave depressions. The tympanic bone is itself moveable, being articulated to the cranium by two diarthrodial joints, so that a bird's lower jaw is swung to the head by two moveable jointings.
.Reptilia. —The evenly concave articular surface of a reptile's lower jaw is often con tributed to by more than one of the osseous pieces which compose the inferior maxilla of the oviparous vertebrates. In the crocodile
by two, in some chelonia by three of these pieces. The tympanic bone is articulated with the other cranial bones by suture, and is therefore immoveable, in the Crocodilia and Chelonia, but it is articulated with them by diarthrosis, and therefore moveable, in the Lacertia, Ophidia, and Batrachia. In some ophidia, as the Python, there being, properly speaking, no symphysis of the lower jaw, but instead of it an elastic ligament, holding the two halves together at the chin, one temporo. maxillary joint is capable of movement inde pendently of the other. In the Batrachia, among their many fish-like characters is that of a dismemberment of the tympanic bone, which consists of an upper and a lower piece.
Pisces. — In the majority of fishes, the tympanic bone is represented by four separate pieces called epi-, hypo-, meso-, and prm tympanic bones. These, further, bear upon their posterior edge three opercular bones, which were considered by Geoffroy St. Hilaire to be the homologues of the ossicula auditus, but are regarded by Prof. Owen as append ages, serially homologous with the costal appendages. The uppermost piece, the epi tympanic, articulates Uy a diarthrodial joint with the mastoid, and the lowermost, hypo tympanic, presents a diarthrodial convexity to the lower jaw. The four tympanic pieces arti culate with one another, and with the oper cular bones by the interposition of ligament, or rather membrane, connecting their thin ad jacent edges together, so that the whole appa ratus is capable of bilging outwards in the movements of respiration. In the eel tribe (illurcenidce) the number of tympanic bones is reduced to three, which is obviously an approach towards the two tympanic pieces of the Batrachia. The formation of the cranial part of the joint in question by the pterygoid and tympanic bones conjointly, in the Lepido siren has been mentioned above. In the Sharks and Rays the tympanic pedicle descends upon that part of the pterygoid which forms the joint, but does not actually reach the articula tion, the pterygoid being interposed. In these fishes, the super-maxillary and pterygoid pieces being confluent, and both bearing teeth, it ap pears at first sight that the whole upper jaw is formed by the former alone, consequently that the inferior maxilla is articulated directly with the super-maxillary ; such an articulation, however, never takes place in any animal.
The interarticular fibro-cartilage is con stantly met with in Manzmalia, but in neither of the other vertebrate classes.
Homology of the joint. — The joint in ques tion is the articulation between the pleura pophysis and the hcemapophysis of the frontal vertebra. It is therefore serially homologous with the joint between the rib and the costal cartilage of mammalia, or the vertebral and sternal ribs of birds and reptiles.
The joints connected with the maxillary apparatus of some of the animals belonging to the three lower sub-kingdoms are, as was stated in the beginning, not homologous with this joint, although identical in function. For description of these parts see the articles