MORPHOLOGY OF THE OWLS. 1.9 



face articulating and iiltimately fusing with a similar flattened area on the parietal 

 and alisphenoid. As a consequence of this inturning of the convex squamosal, 

 a spacioiis chamber is formed — the recessus tympanicus siqjerior. 



A comparison between the skull of the adult and nestling of Syrnium and Speotyto 

 reveals yet other characters of considerable interest ; these concern the morphology 

 of the postorbital process and the tympanic wing, and the formation of the temporal 

 fossa. All three points may conveniently be discussed here. 



Commencing with the postorbital process, we may remark that this, in Syrnium 

 and other forms with a similar type of skull, is formed by the alisphenoid, whilst 

 in Speotyto it is formed by the antero-inferior angle of the squamosal. 



The tympanic wing in both types of skull is formed by the free edges, exoccipital and 

 squamosal. 



The temporal fossa in the young skulls is wanting. It is formed, in Syrnium, by 

 a depression which has for its centre the sutures of the alisphenoid, squamosal, and 

 parietal bones, and later becomes more sharply defined by the excessive development 

 of the postorbital processes and tympanic wing (compare PI. 2. figs. 4, 6). 



J?rom the cranial cavity the squamosal in Syrnium is only visible as a small hour- 

 glass-shaped tract of bone lying between the alisphenoid and parietal and laterad of 

 the pro-otic; bat in Speotyto it forms a large quadrangular plate bounded above by 

 the frontal, below by the pro-otic, behind by the parietal, and in front by the alisphenoid, 

 thus contributing to a very considerable extent to the formation of the brain-case. 



This upward grow^th of tlie squamosal externally, accompanied by the gradual 

 absorption of the underlying frontal and parietal elements and the usurpation of their 

 function in the protection of the brain, is an extremely interesting feature, and marks 

 an advance in the evolution of the skull. Proof of this advance we may derive from the 

 fact that the arrangement seen in Syrnium is a primitive one, agreeing almost exactly 

 with that seen in Dromceus for example. In the latter, it is furthermore interesting to 

 note, the squamosal has not yet succeeded in absorbing the parietal wall, and so is 

 entirely excluded from participation in the formation of the cranial cavity. 



I propose to deal with this question shortly in another communication, in which 

 I hope also to be able to show that, in the evolution of the Avian skuU, a gradual 

 increase in the length of the frontal has taken place, accompanied by a shifting 

 backwards of the parietal and supraoccipital, the last two moving backwards through a 

 quarter of a circle. 



The nasal is truncated posteriorly, and does not extend backwards quite so far as 

 the posterior border of the horizontal plate of the mesethraoid. Mesially the nasals 

 meet one another in Syrnium, and almost conceal the mescthtnoid ; but in Speotyto 

 they only overlap the mesethmoid, leaving the centre of that plate fully exposed. The 

 form of the nasal cleft is holorhinal. The maxillary process of the nasal is truncated 

 inferior ly ; the premaxillary process is long and slender. 



The lachrymal is placed rather far forwards, and lies entirely underneath the nasal. 

 It differs but little from that of the adult stage in form. 



The premaxilla has the nasal processes strongly arched, and terminating at the 



3* 



