[Illustration: FIG. 8.—_Sphenodon_ (tuatera). Skull from side and above: _pm_, premaxilla; _n_, nasal; _prf_, prefrontal; _f_, frontal; _pf_, postfrontal; _p_, parietal; _po_, postorbital; _sq_, squamosal; _m_, maxilla; _j_, jugal; _qj_, quadratojugal; _q_, quadrate; _c_, coronoid; _sa_, surangular; _art_, articular; _pa_, prearticular; _d_, dentary; _an_, angular.] Better evidences of relationships, or the absence of relationships, are offered by the presence of certain bones in the skulls in some orders that are lost in others, since it may be accepted as an axiom that new bones have not appeared in the skulls of reptiles, birds, or mammals; and that no bone which has once disappeared has ever been functionally regained by the descendants of those that lost it. The presence, then, of an extra bone in the temporal region of the lizards or the ichthyosaurs is proof that they have had a long and independent descent from reptiles which possessed it. The mandible of the earliest reptiles was composed of not less than seven separate and distinct bones, as shown in the accompanying figures. The mandible of no modern reptile has more than six, and some have fewer. The mandible of mammals is composed of a single bone, the dentary; those reptiles, the Theriodontia, which doubtless were ancestral to the mammals in Triassic times, have all the bones, except the dentary, much reduced, or even vestigial. The prearticular bone, as shown, so far as known, has been absent in all reptiles since Triassic times, except the ichthyosaurs, plesiosaurs, _Sphenodon_, and turtles, all reptiles of ancient origin. The coronoid bone primitively extended the whole length of the teeth on the inner side; in all reptiles, except the plesiosaurs, since Triassic times it is either reduced to a small bone back of the teeth or is absent. So also the splenial has been greatly reduced in size in all later reptiles and may be wanting as in _Sphenodon_ and modern turtles. The articular of reptiles, it is now generally believed, is represented in mammals by one of the ear bones, the quadrate by another. [Illustration: FIG. 9.—Mandible of _Trimerorhachis_, a stegocephalian amphibian, ancestrally related to the reptiles: _A_ from within; _B_ from without. The coronoid is composed of three bones, the true coronoid (_cor_), the intercoronoid (_icor_), and the precoronoid (_pc_). The splenial is composed of two, the true splenial (_sp_) and the postsplenial (_psp_). The prearticular (_pa_) is broad, the dentary (_d_) is small; and the angular (_an_) is only slightly visible on the inner side.]