In the Reptilia the site of the palato-quadrate bar is sur coronoid ossifications and in some cases a mento-meckellian as well. The quadrate bone with which it still articulates is becoming included in the wall of the tympanic cavity, and, according to H. Gadow, it is this bone and not the para-quadrate which will be come the tympanic of mammals. The hyoid arch is sometimes suppressed in snakes, but in Sphenodon its continuity with the columella or stapes can be demonstrated.
The branchial skeleton is reduced with the cessation of bran chial respiration and only the ventral parts of two arches can be seen; these unite to form a plate with the hyoid (basihyobran chial) and with this the glottis is closely connected. In birds the morphology of the visceral skeleton is on the reptilian plan, and, although the modifications are numerous, they are not of special interest in elucidating the problems of human morphology.
In the Mammalia the premaxilla, maxilla, palate and pterygoid bones can be seen in connection with the region where the palato quadrate cartilage lay in the lower Vertebrata (see fig. 34). The premaxilla bears the incisor teeth, and except in man the suture between it and the maxilla is evident on the face if a young enough animal be looked at. The maxilla bears the rest of the teeth and articulates laterally with the fugal or snalar, which in its turn articulates posteriorly with the zygomatic process of the squamosal, so that a zygomatic arch, peculiar to mammals, is formed. Both the maxilla and palate form the hard palate as in crocodiles, but the pterygoid bone fuses with the sphenoid to form the internal pterygoid plate (see fig. 34, Pt.). The mandible
no longer articulates with the quadrate but forms a new articula tion, by means of the condyle, with the glenoid cavity of the squamosal, and many modern morphologists hold that the quadrate has become the tympanic bone. In many mammals (e.g., Car nivora) this bone swells out to form the bully tympani. The derivation of the auditory ossicles has been discussed in the sec tion on embryology as well as in the article EAR. The presence of a chain of ossicles is peculiar to the Mammalia.
In many of the lower mammals (e.g., Ungulata and Carnivora) the hyoid arch is much more completely ossified than it is in man, tympano-, stylo-, epi-, cerato- and basihyal elements all being bony (see fig. 34). It is of interest to notice that in the hares and rabbits the body of the hyoid has occasionally been found in two pieces, indicating its derivation from the second and third vis rounded by the same series of bones that are found in the Am phibia, but in lizards and chelonians a para-quadrate bone is found which, according to E. Gaupp, is the precursor of the tympanic ring of mammals. In the crocodiles the maxilla and palate grow inwards to meet one another and so form a hard palate. The mandible has dentary, splenial, angular, surangular, articular and ceral arches. The fourth and fifth arches, which form the thyroid cartilage in mammals, are considered in the article RESPIRATORY SYSTEM.