126 SHUFELDT. [Vol. XVII. 



difference in the cranium of the two species is not very 

 marked, it being confined principally to the oblique osseous 

 plane formed by the frontal, upon either side, that extends to 

 form the posterior periphery of the orbit. This is greater in 

 A. otus than in A. accipitriniis. 



Group IV. Ear-flaps present. Cranium symmetrical. Auric- 

 ular openings reniform in outline, and the ear-flap largest 

 upon the right side. 



7. Syrnium aluco (Linn.). 



Auricular openings wide, carried high up, and with broad ear-flaps. 

 Osseous crest on os squamosum completely free above. Viewed anteri- 

 orly, they are almost concealed by the orbit upon either side. Posterior 

 periphery of orbit, where formed by the frpntal, rounded. Median furrow 

 present upon superior aspect of cranium; and the greatest depth of the 

 latter is at a point just over the center of the orbit. Jugal linear. Vomer 

 rudimentary. Supraoccipital foramen present. 



In Syrnium aluco, which belongs to the fourth group, the 

 cranium, ear-openings, and ear-flaps are constructed upon a 

 very different type as compared with the other species. We 

 find the cranium still symmetrical ; but the auricular openings, 

 which are slit high up, and are reniform in outline, are larger 

 upon the right side. The same obtains with the broad, almost 

 door-shaped ear-flaps, the form of which is somewhat different 

 upon either side of the head. 



To this group also belong the other two North-European 

 species — .S. uralcnse and S. lapponictmi. In them the cra- 

 nium is alike in form, and the auricular openings and the ear- 

 flaps are as they occur in 5. aluco. In these species, however, 

 the asymmetry is extended even to include the cranium, while 

 in ^. aluco the cranium is perfectly symmetrical. 5. aluco 

 has the margins of the frontals evenly rounded where they go 

 to form the posterior peripheries of the orbits, and the hinder 

 part of the cranium is prettily dome-shaped, with a conspic- 

 uous, nail-like, anteriorly projecting process upon either os 

 squamosum. The thinner part of the interorbital septum, 

 which until now has been comparatively thick, is beginning 



