112 Prof. J. Miiller un the Structure of the Echinoderms. 



on the flanks, and by the blue colour of the beak and bright 

 yellow of the cere and feet. It is now certain that the conti- 

 nuity or non-continuity of the bars of the tail is not of specific 

 importance^ as I originally thought. No further observations 

 need be made here respecting the young and adult plumages of 

 F. Islandicus, as in my former paper I described from birds shot 

 at their breeding station, having in my possession a brood with 

 their two parents. It therefore only remains to be shown that 

 no change takes place in this species after it has attained its 

 mature garb ; and of this there is now sufficient evidence. 



In August 1847 1 received a living mature male Iceland falcon ; 

 it had not quite completed its first moult, having still a few of the 

 nest feathers, and was a fine characteristic specimen. It died in 

 June 1850, after having cast its feathers three times; and its last 

 plumage was precisely similar to that of its first mature dress — 

 being no whiter nor in the least changed in the markings. 

 Another individual may be alluded to which was brought to 

 England in 1846, and which I saw in November 1848, after it 

 had completed its second moult. It was afterwards sent to the 

 Zoological Gardens, Regent's Park, where I saw it again in the 

 beginning of last year (1853) ; and though it must then have 

 changed its plumage four times since I first examined it, there was 

 no perceptible difference in its whiteness, or in the character of its 

 markings. Other examples might be cited, for I have had many 

 opportunities of seeing this species alive ; but the above would 

 seem quite sufficient to prove that the Iceland falcon undergoes 

 no further change after having attained its mature plumage. 



In conclusion it may be stated that the characters of the two 

 forms are permanent and sharply defined, never blending into 

 each other ; and that the young as well as the mature birds can 

 always be distinguished. But whether these two falcons are to 

 be considered distinct species or mere races must depend upon 

 the views entertained regarding what is to constitute specific 

 character. For my own part I see no reason to doubt the cor- 

 rectness of the opinion I originally expressed. 



XII. — On the Structure of the Echinoderms. 

 By Johannes Muller. 



[Continued from p. 24.] 



Ambulacra of the Asteridse. 



With the restriction of the ambulacra to the ventral surface in 

 the Asteridce, all difi'erences among the feet disappear. They are 

 always locomotive and either conical, as in all Asteridce without 



