MOEPHOLOGT OF THE OWLS. 



33 



the manus has hecome still greater. The accompanying diagram illustrates this. Similar 

 variations in the rate of growth obtain also in the pelvic limb. 



Humerus. ' Manus. 



arm. 



Humerus. Manus. 



arm. 



Humerus. Manus. 



arm. 



Nestlings. 



Adult. 



The pneumatic foramen, so conspicuous in the humerus of the adult, is wanting in the 



nestling. 



IX. The Pelvic Limb. 



The pelvic limb of the Striges is not pneumatic, but otherwise bears a strong 

 resemblance to that of the Falconiformes. It may be distinguished therefrom, how- 

 ever, apart from this character, by the fact that the tibio-tarsus lacks an extensor 

 bridge. 



The femur is relatively long and slender, with a cylindrical shaft. The linea aspera 

 dividing the surfaces for the crureus and vastus externus runs the whole length of the 

 shaft from the ant -trochanter to the base of the internal tibial condyle in the Strigidae ; 

 in the Asionidre it bifurcates near the distal third of the shaft. The popliteal fossa is 

 shallow. The rotular channel in the Bubonidse is relatively shallow, but broad and deep 

 in the Strigidge. 



The tibio-tarsus may be at once distinguished from that of the Falconiformes in 

 that the extensor bridge is conspicuously absent. Ecto- and ento-cnemial crests are not 

 markedly developed. The shaft, in the Asionidse, is long, markedly inflected, and 

 bowed slightly forwards. The internal projects downwards below the level of the 

 external tibio-tarsal condyle, whilst the lateral borders of the posterior trochlear 

 surface are produced backwards and upwards into a pair of prominent ridges, a feature 

 which is especially marked in the Strigidie. The tibial shaft immediately above these 

 ridges is somewhat deeply hollowed, so much so in some genera, e. g. Bubo, Speotyto, as 

 to be saved only by a thin plate of bone from perforating the extensor groove on the 

 other side of the shaft. A small but prominent tubercle projects from the side of the 



SECOND SERIES. — ZOOLOGY, VOL. IX. 5 



