No. I.] THE CRANIUM IN THE OWLS. 133 



openings that are lacking in dermal coverts. The structure of 

 its cranium makes it clear that it belongs to a different type 

 as compared with the other North-European species, even if it 

 does appear in one of the first two of the above-arrayed 

 groups. The cranium, which is symmetrical, has the osseous 

 crest of the os squmnosum terminating in a pointed process, 

 that almost comes in contact with the hinder margin of the 

 alisphenoid, and to which it is united by ligament. Therefore 

 the structure of this crest corresponds to what we find in the 

 right side of the cranium in Syrnium lapponicum. The aural 

 entrance is contracted, quite slit-like, on account of the fusion 

 of the crests with the posterior orbital margin. 



As to the remaining characters it may be said that the cra- 

 nium's greatest depth is found to be at a point situated unusu- 

 ally far forwards, almost in the region of the supraoccipital 

 processes; that the jugal is linear; vomer, rudimentary; and 

 the orbital cavities notably large. 



That these characters of the crania, and the structure of the 

 external auricular openings, can be shown to be present to a 

 great extent in all the species of the same genus, is probable, 

 and the few examinations that I have had the opportunity to 

 make of the heads or crania of non-European species have 

 sustained this. Of the genus Nyctala, in which form the 

 asymmetry of the cranium is most evident, there is, at this 

 time, but a single species known that belongs to the nearctic 

 region, viz. — N. acadica (Gmel.), 1788, with the exception of 

 the circumpolar N. tengnialnii. It is quite probable that the 

 asymmetry seen in the former is also similarly exhibited in 

 the last-named species. In a note in the Proc. Acad. PJiila., 

 1870 (p. 73), Mr. Hale Streets invites attention to the fact 

 that a pair of crania in the collection of the Academy, which 

 were thought to belong to Nyctala acadica, exhibited an asym- 

 metry in their cranial structure which from the description 

 corresponds with that seen in N. tengniahni. 



The second species exhibiting asymmetry in its cranium, 

 Symitim lapponicum, is represented in the nearctic region by 

 a related species, 5. cinereiim (Gmel.), 1788, which was de- 

 scribed one year earlier than S. lapponicum. That this species, 



