Arctic and North- Atlantic Starfishes. 401 



distinct, liowever — with a disc-diameter of Ifi-lU mm., wliicli 

 must correspond to Dr. Mortenseti's of 15-20 mm. from Jan 

 Mayen ; the majority in this jjroup cannot be niore than three 

 years oKl. Filially, there is the specimen of 11 mm., the age 

 of which must be estimated at two years. 



The oj)hiurids from the great depths of tlie North Atlantic 

 collecteil by the * jMiehael Bais' in 1910 show that the same 

 conditions exi.st as in the Norwegian Sea. A species of 

 ophiurids may be represented at a locality by several year- 

 classes, but only one of them nunieiously. 



Durinp; the examination of the echinoderms collected by 

 the ' Alichael Sars' in 1010, I also commenced work on the 

 starfishes to see whether the conditions were the same as in 

 the oj)hiurids. In order to iiave material as abundant as 

 possible I did not confine my attention to that taken in 1910, 

 but added some from other cruises of the ' Michael Sars' and 

 similar expeditions. Altogether I examined 14 species, 

 which were collected in Arctic waters, oiF the Norwegian 

 coasts, and in the North Atlantic. I had abundant material 

 of some species : thus of about 500 s|)ecimens of Pontaster 

 tenuispinus 155 were from one locality, and of about 800 

 specimens of Ctenodiscus crispntus 355 were from one locality ; 

 of the remaining species the material was somewhat scant}' — 

 it is, however, of importance for the purpose of comparison. 

 The result of these investigations is more fully discussed i\\ 

 " Nogen asteriders alder og aarsklasser "* ; as this paper is 

 written in Norwegian, I here give an abstract from it. 



The diameter of the disc served to determine the annual 

 classes of the ophiuiids, but in the starfishes it was more 

 convenient to use the radius of the disc. 



The measuring method requires many examples, and even 

 then the age of a specimen can oidy be determined with 

 certainty if its size comes within the measurements of the 

 majority of the individuals of a year-class. Another drawback 

 to this method is that the conditions of life and, consequently, 

 growth may differ at different localities; the results from one 

 locality can therefore not be applied unconditionally to 

 another. Following the usual practice when studying fishes, 

 I have examined the marginal plates of the starfislies and 

 other parts of their skeleton in order to arrive at the age of 

 each individual, but I have not definitely succeeded in finding 

 annual rings. I have therefore been obliged to fall back 

 upon the measuring method. 



• ' Bergens Museums Aarbok,' 1916-17, Naturvidensk Roekke, no. 1. 



