342 ANATlDiE. 



like the parent birds, and did not assume, at any age, the 

 grey colour borne for the greater part of the first two years 

 by the young of the other species of Swans, induced the 

 Author to consider this Swan entitled to rank as a distinct 

 species, and, in reference to the unchangeable colour of the 

 plumage, he proposed for it the name of Gygnus immutahilis. 



The organ of voice appeared, from one that the Author 

 examined, to be like that of the Mute Swan ; but his 

 opinion as to the specific distinctness of the Polish Swan 

 was strengthened by a paper pointing out some differences in 

 the form of the cranium, published by Mr. Pelerin (Mag. 

 Nat. Hist. 1839, p. 178), from which the following is an 

 extract : — 



" The measurement of an adult cranium of each is as 

 follows : — Length, from the tip of the bill to the base of 

 occipital bone in C. immutahilis, six inches and three- 

 eighths ; C. olor, six inches and seven -eighths. Height, 

 from the bottom of the lower mandible when closed, to 

 the top of the protuberance at the base of the bill, in C. 

 iviniutabilis, one inch and five-eighths ; C. olor, two inches. 

 Height, from the base of the under jaw to the vortex of the 

 head, just behind the orbit of the eye, in C. immutahilis, 

 two inches and one-eighth ; C. olor, two inches and a 

 quarter. In C. immutahilis the bill is rather more flattened, 

 particularly in the middle, between the dertrum, or nail, and 

 the nostrils ; the protuberance at the base of the upper 

 mandible is less developed. In the Polish Swan the cranium 

 is highest at the supra-occipital portion ; in the Mute Swan 

 the cranium is highest at the supra-orbital portion ; but the 

 greatest difi'erence is perceptible on comparing the occipital 

 bones ; the upper portion of this bone in C. immutahilis 

 protrudes considerably more, and there are two oval fora- 

 mina, one on each side just above the foramen magnum, 

 which are not present in any specimens of C. olor that I 

 have examined ; the portion forming the boundary of the 

 external orifice of the ear is much more prominent, and the 

 condyle forms a more acute angle with the basilar portion 

 of the occipital bone." 



