412 THE OWLS 



tawny-owls, Syrnium, this aperture is not only relatively much larger, 

 but the skin bounding its anterior margin has been produced to form 

 a broad flap or " operculum," which can be drawn over the aperture, 

 closing it completely (Fig. 2, Op.). Further, if the two sides of the 

 head be carefully examined, it will be found that the aperture on the 

 right side of the head is larger than that of the left ; and, as we shall 

 show presently, this asymmetry is by no means confined to the species 

 of this genus. The aperture is so large that the side of the skull-wall, 

 and the sclerotic plates of the eye, are exposed when the oper- 

 culum is raised, the actual meatus, or opening of the ear lying in 

 the lower segment of the great oval opening formed by the skin. 

 (Figs. 2 and 3, M.E.). In the long and shorteared-owls the size of this 

 opening attains its maximum. In the owls of the genus Otus the 

 inferior border of this aperture is continued downwards and forwards 

 to the gape, and upwards, backwards, and forwards in a great sweep 

 to the top of the eye, and a correspondingly enormous operculum 

 is developed in consequence. (Fig. 1.) But this is not all. From the 

 middle of the inner surface of the operculum there runs backwards to 

 the thin sheet of skin covering the cranial wall a long pulley ensheathed 

 in a band of skin. Thus on raising the operculum a great space 

 is revealed, exposing a large part of the cranium, the hinder part of 

 the lower jaw, and the hinder wall of the eyeball. Behind the latter 

 is a large crescentic cavity divided into an upper and a lower chamber 

 by the band of skin running backwards from the operculum. Care- 

 fully examined, this upper chamber on the right side of the head will 

 be found to be "blind," while in the floor of the lower chamber will 

 be found the meatus already referred to. On the left side of the 

 head the relations of these chambers is exactly reversed, the meatus 

 opening into the chamber above the pulley. Here, again, we have an 

 instance of asymmetry. What advantage the extraordinary develop- 

 ment of the external ear confers, or what function it performs, is so far 

 an insoluble riddle. It would seem, and this is merely supposition, that 

 this curious development forms a kind of ear-trumpet, for the hinder 



