MOEPHOLOaT OF THE OWLS. 259 



The radii — like the rami — are transparent. Fila are generally but not always present. 

 The rami are laterally compressed. Structurally they can scarcely be distinguished from 

 those of Falco tinnunculus ; but in this latter the fila are longest and regularly arranged 

 along the radius (fig. 3, D). 



The pre-pennse of Capriniulgm can readily be distinguished from either Speotyto or 

 Falco tinminculus. As in these, they are umbelliform, but the rami are produced con- 

 siderably beyond the most distal radii into long filamentous processes, and the radii bear 

 minute fila at regular intervals (fig. 3, B). 



The pre-plunuda; differ from the pre-pennae, both in Speotyto and the Kestrel, in that 

 the radii are divided np into segments by the collection of pigment in the region from 

 which the fila arise. In this respect they resemble the definitive down-feathers, but can 

 always be distinguished from them. At least this is true of the Kestrel. Thus, in this 

 species, the pigment, though in the form of a dense black deposit in the region of the 

 fila, shows a tendency to be distributed all along the radius (fig. 3, C). Eurther, the 

 pre-plumulse are umljelliform, whilst the definitive plumulae have a rhachis and after- 

 shaft. From the fact that the feathers which I have doubtfully described as pre- 

 plumulae in Speotyto have a distinct rhachis, and the radii are indistinguishable from 

 those of the adult, it is more than probable that these feathers are really definitive 

 down-feathers. 



Both the pre-plumula3 and the definitive down-feathers of Speotyto — if these, in this 

 case, are not one and the same — can be distinguished from the plumulae of the Kestrel in 

 that, in the latter, the segments of the radius are more numerous (fig. 3, E) and the 

 rhachis is longer and supports an aftershaft. 



VII. The External Ear. (Plates 27-28). 



The external aperture of the ear of birds is usually concealed by feathers. If these 

 be raised, a small round or oval opening will be seen leading into a more or less spacious 

 chamber — the outer portion of the auditory meatus. This chamber is continued inwards 

 in the form of a tunnel to the tympanum — the boundary-Avall between it and the middle 

 e;ir. It will be helpful, for clearness' sake, in the present paper to call the cavern-like 

 chamber communicating directly with the outer world the Cavernum, and the tubular 

 inner continuation the Cavornulum. 



In Athene noctua we have apparently one of the least specialized meatuses of all the Owls. 

 The external opening is almost circular in form and very small, its vertical axis being 

 about half the height of the horizontal axis of the closed eyelid. The cavernum is 

 spacious, and leads backwards and downwards to pass by insensible gradations to the 

 cavernulum. In other words, it represents the typicalimspecialized form of the external 

 ear in birds. Erom this ue can pass by a series of slight gradations to forms of con- 

 siderable relative complexity, as will presently be shown. 



"What has been described for Athene will answer almost equally well for Speotyto. 



Bubo lacteus leads us a step further ; at the same time it represents the simplest 

 form of the external ear to be found in the whole genus save that of B. nycteus. The 



