MOEPHOLOGT OF THE OWLS. 7 



it noted, the squamosal wing of the tympanic cavity just described. Above this 

 prominence lies a broad, shallow, linguiform temporal fossa, which, in some forms, to be 

 presently described, may be absent. 



Regarding the tympanic cavity of Syrniuin as representing, at least approximately, 

 the archecentric {of. P. C. Mitchell, Trans. Linn. Soc, ser. 2, Zool. viii. (1901) 

 p. 181) condition, let us turn now to a consideration of the more important apocen- 

 tricities which may be traced therefrom. These are of two kinds : («) symmetrical, 

 and (h) asymmetrical. Amongst the symmetrical forms the departure from the arche- 

 centric type begius by the formation of large bullae derived by an increase in the size of 

 the squamosal wing which forms the posterior tympanic wall, and which may, as in 

 Scops asio, S. rutilus, and S. semitorques, fuse by its superior external angle immediately 

 behind the base of the postorbital process, thus bridging the temporal fossa. By this 

 inflation of the tympanic wing a very spacious cavern is formed, bounded posteriorly 

 and superiorly by the squamosal and exoccipital wings, anteriorly by the postorbital 

 process. It is divided into two more or less well-marked chambers, an outer and an 

 inner, one leading directly into tlie other, the division being bounded by the quadrate. 

 In Gymnasia and Speotyto, by the way, the squamosal wing joins the postorbital process 

 directly, and thus not only bridges the temporal fossa but also roofs in the tympanic 

 cavity. The size of this wing not being increased, however, the resulting outer chamber 

 is very small. 



The form of the tympanic cavity in Asio is still further modified. It appears to be a 

 fvirther specialization of the type seen in Photodiliis and Strix (see p. 9). The squamosal 

 and exoccipital moieties of the tympanic wing are here combined to form an oatstanding 

 wing of bone extending upwards and forwards to terminate above the base of the post- 

 orbital process, from which it is divided, however, by a low, saddle-shaped, ridge. The 

 exoccipital moiety of the wing curves downwards and forwards so as to form a spacious 

 antechamber leading directly to the aj^ertures of the recess/ts tpnpanicus superior and the 

 meatus externus. The tympanic cavity in front is bounded by the postorbital process, of 

 which more anon. Above, the cavity is bounded only by a low ridge connecting the 

 angles of the postorbital and squamosal Avings of the tympanic cavity just described. 

 The deep cavity enclosed, by these wings corresponds to the " posterior division of the 

 cavernum " of the first section of this memoir. For brevity's sake it may be axUed the 

 post-cacernum. The post-cavernum of Asio dilfers from that of Speotyto, for example, 

 mainly in its greater upward extent, in its relatively greater width, and in that it is less 

 perfectly closed above. But Avhilst the tympanic wing of Speotyto forms a consincuous 

 laterally expanded bulla, the wing in Asio forms rather a vertical flattened plate, the 

 inferior portion of which, however, is bullate. This change in the form of the tympanic 

 wing has resulted in an interesting modification — to wit, the suppression of the temporal 

 fossae — and fresh attachments for the first portion of the temporalis muscle. The 

 temporal fossa is in consequence obliterated in Asio. 



In Speotyto, Scops asio, Scops rutilus, and Scojjs semitorques, it must be remembered, the 

 tympanic wing has also extended upwards ; but in these cases the temporal fossa is not 

 suppressed, the wing being perforated to allow of the passage of the muscle. 



