8 ME, W. P. PYCRAFT ON THE 



We must turn now to a consideration of the asymmetrical tympanic cavity as exhibited 

 in the skull of Nyctala Tengmalmi. This appears to be a modification of the type seen in 

 Speotyto, the squamoso-occipital tympanic wings forming large bullse. 



On the left side of the skull (PI. 2. fig. 1 a) the tympanic wing has grown upwards and 

 forwards to join the postorbital process, bridging the temporal fossa as in Speotyto, 

 But with this difference, the fossa is reduced to a mere vestige in the shape of a narrow 

 groove deep enough perhaps to lay a horse-hair in ; this groove passes between the 

 jim^ction of the tympanic wing and the postorbital process through a minute hole into 

 the tympanic cavity. I have not been able to dissect out the muscles of the head, but 

 when this is done it will probably be found that not more than a vestige of the first 

 portion of the temporal muscle remains ; possibly even this has disappeared. The cavity 

 enclosed by the squamoso-parietal wing and the postorbital process corresponds to the jsos^- 

 cavernum of Asio. It is considerably larger than the corresponding cavity in Speotyto, 

 and curves downwards and forwards, so as to form at the same time a shield for, and a 

 spacious antechamber leading to, the apertures of the recessus tympanicus superior and 

 the meatus externus. In the nature and function of the exoccipital moiety of the 

 tympanic wing, Nyctala, it should be remarked, agrees also with Asio and Speotyto, for 

 example. But the tympanic wing of the left side in Nyctala differs from that of any 

 other of the forms with which it has been compared in the development of a large 

 tongue-shaped plate of bone which projects from the middle of the tympanic wing, 

 forwards and downwards, to terminate just below and outside the articulation of the 

 quadrato-jugal bar with the quadrate. 



The tympanic cavity of the right side of the skull in Nyctala differs conspicuously 

 from the left, in that the squamosal portion of the tympanic wing extends upwards to 

 within a short distance of the crown of the head, and lies far behind the base of the 

 postorbital process. Near the middle, the free border of this wing, as on the left side, is 

 drawn out into a tongue-shaped process, which curves inwards at its tip to join the 

 inferior end of the j)Ostoi"bital process. Thus, on this side the articulation of the 

 quadrato-jugal with the quadrate is fully exposed. 



If the post-cavernum of the right and left sides be compared, it will be found that that 

 of the right side has a very large oval aperture, the inferior border of the rim of which 

 lies considerably above the level of the orbital process of the quadrate. The cavity itself 

 may be likened to a deep pocket, which runs directly backward. The aperture of the 

 left side is incomplete, a large gap being present in its lower border, owing to the 

 failure of the tongue-shaped process of the tympanic wing to reach the postorbital 

 process. Eurthermore, the aperture is narrower, crescentic in shape, and terminates 

 superiorly at the level of a line passing forwards from the upper limb of the crescent to 

 the base of the beak. Thus its upward extent is very considerably less than on the right 

 side. A comparison of the figures (PI. 2. fig. 1, 1 a) will make this plain. 



The mesial wall of the pjost-caverntim of the right side, it may be mentioned, is marked 

 by a shallow groove which, traced upwards, is found to lead to a minute aperture, 

 corresponding to the point of union of the tympanic wing with the base of the post- 

 orbital process of the left side. But here, the tympanic wing having shifted backwards, 



