OCR Text |
Show 1902-] OSTEOLOGY OF THE FALCONIFORMES. 281 in the collection of the British Museum the left process is smaller than the right, and both almost fail of their purpose, not only in this species but throughout the genus. They present within the Order every degree of degeneration, from the prominent pillars of Serpentarius to the most minute prickles. Traces of the anterior basicranial fontanelle are often present. The Lateral Surface of the Cranium. (Plate XXXIII. figs. 1-8.) The tympanic cavity attains its greatest size in the Falcons. It may be studied in its more primitive form in such examples as Polyboroides, Pandion, Pernis, and the Vultures. The lateral occipital wings which bound this cavity behind in Polyboroides are but feebly developed. The free border, on either side, terminates in a blunt and low prominence behind and slightly above the level of the processus zygomaticus, and marks at the same time the lower and hindmost extent of the temporal fossa. In Pandion the free border of this wing, which is still feebly developed, slopes obliquely backwards and upwards to the level of the processus articularis squamosum, then turns sharply forwards to terminate in the process itself. Pernis resembles Pandion in this respect, but neither the backward slope nor the forward angle is so marked. In Serpentarius and the Vultures, e.g. Neophron, Gypohierax, and Gypaetus, the free edge of this wing is of greater extent, and projects as a slightly raised ridge beyond the zygomatic process. [In Gyps the form of the lateral occipital wing is drawn backwards and downwards to terminate in a prominent processus alse exoccipitalis inferior (paroccipital process), and thus in this respect differs from Neophron, where the wing is more feebly developed and slopes from before backwards and upwards.] In Buteo the ridge laterad of the zygomatic process is much more prominent. In Haliaetus and Aquila it has increased still more. In the Falcons it forms a thin laminate plate, rising upwards to the level of the base of the zygomatic process of the squamosal, the free edge of which is turned forwards. In the development of this portion of the exoccipital it bears a strong resemblance to the Striges, e. g. Bubo. The roof of this cavity is formed by the under surface of the processus zygomaticus squamosi. Its floor, in part by the lateral occipital wing, and in part by the ossification of tissue extending between this wing and the external angles of the basi-temporal plate. In front it is bounded by the quadrate. Within this cavity, in the dried skull, are two large apertures which may be considered separately. The first lies near the roof of the cavity and separates the squamosal and otic articular surfaces of the quadrate. The aperture leads upwards and backwards, so as to form a space between the anterior vertical and horizontal canals of the internal ear and the lateral occipital wing. This is the recessus tympanicus superior-the temporal recess of m y earlier papers. The second and lower aperture is divided from the first by the PROC. ZOOL. Soc-1902, VOL. I. No. XIX. 19 |