IN THE CRANIA OF. THE BRITISH SWANS. 179 



comparing the occipital bones; the upper portion of this bone 

 in Cyg. immutabilis (a) protrudes considerably more, and 

 there are two oval foramina (b), one on each side just above 

 the foramen magnum, which are not present in any specimens 

 of Cyg. olor that I have examined ; the portion forming the 

 boundary of the external orifice of the ear (c) is much more 

 prominent, and the condyle forms a more acute angle with 

 the basilar portion of the occipital bone. 



Skull of Cygnus immutabilis. 

 A, upper portion of occipital bone. B, oval foramina, C, external orifice of ear. 



One of the chief distinctions of Cyg. olor appears to be the 

 small foramen on the top of the cranium, which 1 have inva- 

 riably found in this, but never in a single instance in the three 

 other species : the two foramina in the occipital bone do not 

 appear so constant, as, although I have always detected them 

 in Cyg. Bewickii and never in Cyg. olor, they are occasional- 

 ly observable in Cyg. ferns, but wanting in the greater num- 

 ber, and even in some, though perceptible, nearly obliterated. 



The small size of the head of Cyg. Bewickii, being usually 

 about one third less than that of Cyg. ferns, and the compa- 

 rative shortness and breadth of the bill, render it unnecessary 

 to describe it more minutely, as it would be at once detected 

 by any observer at all conversant with the subject. 



The specimen of Cyg. immutabilis from which the accom- 

 panying drawing and description are taken, was procured in 

 the London market, and has been some years in my collec- 

 tion ; and from a frequent comparison with many crania of 

 Cyg. olor, from which it uniformly differed in the above par- 

 ticulars, I considered it as the head of a nearly-allied but dis- 



