SHALLOW-WATER STARFISHES 41 



albula? which is a very widely distributed North Atlantic and Arctic 

 species, which has been found at moderate depths off the northern 

 Alaskan coasts. I consider it one of the Asteriinae, allied to Lept- 

 asterias. 



It is autotomous while young, with a variable number of rays, but 

 when large it is usually regularly six-rayed. 



The consideration of the importance of the stichasterial or imbri- 

 cated arrangement of the dorsal ossicles early led to the separation of 

 Stichaster as a genus, and later as the type of a family, but perhaps 

 even that character does not always indicate close affinity nor an 

 identical origin, for it may have been developed independently in 

 different regions and in distinct lines of descent. As a matter of 

 fact, we find the greater number of the species, and those that are 

 the larger and more typical, living in the Antarctic seas and on the 

 southwestern coast of South America, while only a few rather small 

 and less characteristic forms exist in northern seas. 



It is not unlikely that the northern forms, like S. rosea and 5*. 

 albula, originated entirely independently of those of South American 

 seas, and should, therefore, be classed as distinct genera. This is 

 indicated, moreover, by their morphological characters. S. albula, 

 especially, differs but little from some forms of Lepasterias, and may 

 well have been developed from some member of that or a similar 

 group. Its embryology is not known. 



However, all recent investigators admit that the stichasterial 

 arrangement of the dorsal ossicles is at least of generic value.* 



Other variations in these plates may be of equal value. 



The discovery of many new generic and specific types intermediate 

 between typical Stichaster and Asterias, as already intimated, renders 

 it difficult to define the limits of the two so-called families, typi- 

 fied by these genera. 



'This is the Asterias albula (Stimp.) = Stichaster albulus Ver. = 

 asterias albula Ver. = Stichaster albulus S\3iden = Nanaster albulus Perrier, 

 1894, p. 131. It is found from the Arctic Ocean to South Carolina and Georgia, 

 in deep water, on the east American coast, and as a littoral species it occurs 

 as far south as the Bay of Fundy. It is also common on the northern Euro- 

 pean coasts. 



2 Sladen has even gone so far (op. cit., p. xxxvi, 1889) as to widely separate 

 Stichasteridae and Zoroasteridae from the Asteriidae on this character, almost 

 exclusively, placing them between Linckiidae and Solasteridae families that do 

 not belong to the Forcipulata. This seems to me a very unnatural arrange- 

 ment, due to overestimating the importance of the character of the dorsal 

 ossicles. 



