OCR Text |
Show 288 MR. W. P. PYCRAFT ON THE [Apr. 15, form, with the nasal bones, a more or less perfect fronto-nasal hinge, see also p. 291. There is no transverse hinge as in Microlvierax or in Parrots. The Maxillo-Jugal Arch. The maxilla is indistinguishably fused with the premaxilla. The maxillo-palatine processes have slowly increased in size so as to convert what was originally a schizognathous into a desmo-gnathous palate. Many intermediate stages in the course of this transformation have been preserved and will be discussed forthwith. In the study of the transition of the palate from the schizognathous to the desmognathous type, we will commence with Elanus : this form being regarded by Dr. Suschkin as exhibiting more primitive characters than any other Accipitrine bird. In Elanus then (PI. X X X I . fig. 2) the maxillo-palatine processes turn inwards in the form of a pair of small, very pneumatic, hamulate processes. Although they hem-in the vomer on either side, they do not extend so far inwards as to touch it. Distad of the vomer they are hidden by the palatines, and thus leave a large median palatal vacuity exposing the nasal septum above and lying in the middle line. The maxillo-palatine processes themselves are pierced, laterad of the palatines, and between these and the tomium, by a large foramen. The palate, however, of Elanus is not, I think, to be regarded as primitive, but specialized (see p. 313). Circaetus (PI. X X X I . fig. 4) is also schizognathous, and probably represents the intermediate type of palate which has given rise, by specialization, in one direction to the palate such as is found in Elanus and the Cathartse, and in the other to such as obtains in the Vultures and Falconidse. In Aquila (e. g. A. chrysaetos, A. audax), Spilornis, and Thras-aetus harpyia we find the maxillo-palatines greatly increased in size antero-posteriorly, and extending inwards mesiad of the palatines : so much so as to almost completely fill up the space between them. The nasal septum is now visible only through a narrow palatine clink. This increase in bulk has now caused the maxillo-palatines so nearly to approach one another, that they almost, if not quite, touch. But they are yet distinct, and the palate must accordingly be regarded as schizognathous. In Serpentarius they touch throughout the greater part of their length, concealing the nasal septum (PI. X X X I . fig. 7). Indirect desmognathism, brought about by the downward extension of the nasal septum, obtains in the majority of Accipitres, and can best be studied in such forms as Elanoides, Pandion, Vultures. In Gypaetus and Neophron, for example, the maxillo-palatines do not meet in the middle line, and but for the nasal septum would be bounded anteriorly by a large palatal vacuity as in Cathartes. Direct desmognathism is perhaps only found in Falconidse. |