OCR Text |
Show 1902.] OSTEOLOGV OF THE FALCONIFORMES. 309 deep in the remaining forms, whilst the ossified extensor bridge is conspicuous by its absence. The absence of this bridge should surely be as valuable a piece of evidence that Pandion is not related to the Striges, as is the presence of an extensor bridge on the tarso-metatarsus to show that it is so related ! Furthermore, we may remark that this tarso-metatarsal bridge is wanting in the Barn-Owls, so that the value of this character is still further weakened. The tibio-tarsus of Polyboroides is, in its way, almost as remarkable as that of Pandion. It is long and almost of the same thickness throughout. As in Pandion, the width of the shaft immediately above the distal condyles is greater than across the condyles themselves. This is probably clue to the fusion with the shaft of the distal end of the fibula which extends down to the tarsal elements-though, as just indicated, fused with the shaft. The extensor bridge is placed transversely across the shaft. The tibial condyles are but feebly developed, the ectocondyle barely projecting beyond the shaft. The intercondylar gorge is wide. The lower third of the shaft of the larger Accipitres is dorso ventrally depressed. There is often an additional and well-defined fibular bridge. The second lies some distance below the first, and affords attachment for the distal end of the fibula. The cnemial crests are relatively but slightly developed. In Aquila, perhaps, they reach the maximum development, the entocnemial crest being unusually strong. In Thrasaetus the ecto- and entocnemial condyles are conspicuously wide apart, causing the inner border- of the shaft to have a very marked curve. The fibula, as a rule, tapers to a fine point, and terminates near the lower third of the tibial shaft, sometimes fusing therewith. In Pandion, Polyboroides, and Pernis only is it of almost uniform thickness and continued downwards as far as the tarsal elements. The tarso-metatarsus varies extremely, both in its relative length and in the development of bony matter for the mechanical requirements of the limb. In Serpentarius only is the tarso-metatarsus as long as the tibial shaft. In Accipiter, however, it is very nearly so. Generally it is shorter than the femur. The hypotarsus is simple in all save Pandion and Pernis, wherein it forms a tube. The distal trochlese, save in Serpentarius and Cathartse and in Leptodon, are all on the same level. In the two first-mentioned exceptions the middle trochlea is produced somewhat beyond the level of the others. In Leptodon the inner trochlea is the longest, the middle slightly shorter, and the outer shorter still, so that an obliquely sloping series is formed. The plane of the trochlese forms a slight and regular curve, except in Pandion, in which this curve is very strong; and Polyboroides, in which the 2nd and 3rd trochlese lie close together, whilst the 1st is bent downwards so |