178 STRUCTURAL DIFFERENCES 



Art. VII. — On the structural differences observable in the Crania 

 of the four British Species of the Genus Cygnus. By W. G. 

 Pelerin, Esq. 



It may be considered by many that structural variation in 

 the crania of birds is of little importance ; but I am inclined 

 to think, from an attentive study of the comparative anatomy 

 of the class for some years, that it may frequently tend, if not 

 absolutely to decide species, at least materially to strengthen 

 the characters. Many species that were long considered dif- 

 ferent, as the purre, dunlin, &c, had the crania been minute- 

 ly examined, would have proved identical, and vice versa, as 

 in the present instances, where the birds are so nearly allied 

 in point of colour &c. as to have been for years confounded, 

 it appears to me worthy of attentive consideration. 



As some doubts have been lately promulgated on the con- 

 tinent with regard to the swan characterised by Mr. Yarrell 

 being specifically different from Cygnus olor, a description 

 and comparison of the cranium of each may be interesting to 

 naturalists, and tend to prove satisfactorily that they are dis- 

 tinct. 



The measurement of an adult cranium of each bird is as 

 follows. 



Length from the tip of the bill to the base of the occipital 

 bone : — 



Cyg. immutabilis, 6f in. Cyg. olor, 6| in. 



Height from the bottom of the lower mandible when closed 

 to the top of the protuberance at the base of the bill : — 

 Cyg. immutabilis, If in. Cyg. olor, 2 in. 



Height from the base of the under jaw to the vertex of the 

 head, just behind the orbit of the eye : — 



Cyg. immutabilis, 2^ in. Cyg. olor. 2\ in. 



In Cyg. immutabilis the bill is rather more flattened, par- 

 ticularly in the middle between the dertrum and the nostrils ; 

 the protuberance at the base of the upper mandible is less de- 

 veloped, and the top of the skull, instead of being nearly on 

 a level from thence to the summit of the parietal bones, as in 

 Cyg. olor, rises gradually to that point, and does not indicate 

 any sign of the small foramen observable at that part in the 

 latter species : but the greatest difference is perceptible on 



