Arctic and North-Atlantic Starfishes. 401 
distinct, however—with a disc-diameter of 16-19 mm., which 
must correspond to Dr. Mortensen’s of 15-20 mm. from Jan 
Mayen ; the majority in this group cannot be more than three 
years old. Finally, there is the specimen of 11 mm., the age 
of which must be estimated at two years. 
The ophiurids from the great depths of the North Atlantic 
collected by the ‘ Michael Sars’ in 1910 show that the same 
conditions exist as in the Norwegian Sea. A species of 
ophiurids may be represented at a locality by several year- 
classes, but only one of them numerously. 
During the examination of the echinoderms collected by 
the ‘ Michael Sars’ in 1910, I also commenced work on the 
starfishes to see whether the conditions were the same as in 
the ophiurids. In order to have 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, off the Norwegian 
coasts, and in the North Atlantic. I had abundant material 
of some species: thus of about 500 specimens of Pontaster 
tenuispinus 155 were from one locality, and of about 800 
specimens of Ctenodiscus crispatus 355 were from one locality ; 
of the remaining species the material was somewhat scanty— 
it is, however, of importance for the purpose of comparison. 
The result of these investigations is more fully discussed in 
“ 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 ophiurids, 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 only 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, 
IT have examined the marginal plates of the starfishes 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 Rekke, no, 1, 
