Thus far the ossification of the superficies of the skull of Archegosaurus closely conforms to that of the salamandroid ganoid fishes above cited ; and the homologous bones are determinable without doubt. The lacrymal bone obviously answers to the front large suborbital scale-bone in fishes ; its large size and forward extension in Archegosaurus is a mark of that affinity.
The upper jaw consists of pre-maxillary (zz), minrillary (zi), and palatine bones. The pre-maxillaries are divided by a median suture, as in Lepidosteus and Crocodilus, and are short bones, the breadth exceeding the length in A. latirostris, and also in the young of A. Decheni ; but in the old animal opposite proportions prevail In A. Decheni each pre-ma-Till/try contains eight teeth ; in A. latirostris not less than eleven.
The maxillary (21) which extends from the pre-maxillary to beneath and beyond the orbit, presents a great length, varied according to species and age ; it is of small vertical extent, and terminates in a point, which reaches the tympanic. An teriorly it unites with the pre-maxillary, and enters into the formation of the back boundary of the nostril ; mesially it unites above with the lacrymal and suborbital, and below forms the outer boundary of the choanal aperture, joining the vomer anteriorly, and the palatine posteriorly. The palatine is a long narrow bone, rather expanded at both extremities ; it forms anteriorly the hinder border of the choanal aperture, and mesially throughout a great part of its extent the outer boundary of the great palatal vacuity. It supports a row of teeth, of which one or two at the fore part are of large size.
Between the orbit and the maxillary extends an oblong flat bone (26), forming the lower or outer border of the orbit, uniting with the pre-frontal and lacrymal anteriorly, with the maxillary below, and with the tympanic ( 9 ) and another bone behind. In this position, and in its connections, it agrees with the malar of the crocodile, and also with the suborbital bone or bones of fishes. The latter are unequivocally muco dermal bones, and may not be the homologues of the endo skeletal malar bone of saurians, birds, and mammals. To which of the bones, therefore,—suborbital or malar,—the one in question of the Archegosaurus answers, may be doubtful. The writer inclines to view it as a dermal ossification, and to conclude that, as in the higher Batrachia, the true malar and the zygomatic arch are not developed. Admitting the doubt
on this point., the bone (26) may be termed the " suborbital." With regard to the next bone (27), the same question, whether it answers to the squamosal in the crocodile, or whether it is a dermal ossification, applies. If a homology with a determinate endo-skeletal bone in the crocodile and higher vertebrates were to be predicated, it would be the " squamosaL" Above 27, between it and 8, is the " supra squamosaL" Essentially it indicates the tendency to excessive dermal ossification of the skull, like that which extends into the superficial temporal fascia from the squamosal and mastoid in the Chelonia ; this separate ossification in Arch.egosaurms roofs over the temporal fossa. It is the homologue of the supernumerary surface-bone called " supersquamosal" in the Labyrinthodonts ; and both this and the "postorbital" corre spond in position with the posterior suborbital scale-bones in Amia and Lepitiosteus.
The hinder angles of the skull are formed by the tympanic; in young individuals the tympanic does not extend backward beyond the par-occipital, but as age advances it projects further backward. It appears to abut internally against the pterygoid.
The two rami of the mandible were loosely united at a short symphysis, not exceeding the breadth or depth of the jaw at that point ; the depth gradually augments to near the articular end, but never exceeds a sixth, and is usually only an eighth of the length of the jaw, no definite coronoid process being developed ; the upper and lower borders are nearly straight as far as the deepest part. The lower border behind this part rises rather abruptly to an angle, which is just below the articular pit. The angular element (30) presents a con vexity answering to the point of ossification whence some faint ridges radiate upon its outer surface. The dentary (30, if it does not form the articular surface, begins very near it and each ramus appears to be composed of these two bones. The dentary developes the coronoid rising. Neither articular nor splenial element has been clearly demonstrated. If an articu lar element has existed, it has been very small.