Arctic and North-Atlantic Starfishes. 407 
represent two or more, only one of them being numerous. 
The material also shows that each year-class is grouped about 
certain definite sizes, and that these as a whole are common 
to all species examined. Pontaster tenuispinus has a year- 
class whose maximum is gathered about a disc-radius of 
3-4 mm.; Ctenodiscus crispatus, Leptychaster arcticus, 
Bathybiaster veaillifer, Solaster papposus, Hymenaster pellu- 
cidus, Henricia sanguinolenta, etc., have likewise a year-class 
of this size ; Pontaster tenuispinus has the next year-class at 
8-10 mm. In like manner we find that Plutonaster bifrons, 
Dytaster agassizi, Ctenodiscus crispatus, Hymenaster pellu- 
cidus, etc., have a year-class at the same size. It should be 
noted that the figures are not absolutely exact ; with respect 
to some species and localities they may be found a little above 
or a little below those given, as I have already stated. 
Speaking generally, these are agreements which cannot be 
due to accidental circumstances. Hach of these year-classes 
must have a fixed age, and if the year-class 3-4 mm. of 
Pontaster tenuispinus is one year old, then that year-class 
of the other species which is of the same size must also be 
one year old. It was therefore important to fix the age of a 
year-class in a species, as thereby the age was given for the 
other species. 
I was fortunate to determine the age of some of the speci- 
mens of Psilaster andromeda, collected by Dr. D. C. 
Danielssen at Utne, Hardanger. The smallest specimens 
from Utne have a disc-radius of 0°5 mm. ‘They were young 
which had recently been transformed to the bottom stage, 
and thus belong to group 0. Of the rest two specimens 
measured 3 mm., three 7-8 mm., four 10-12 mm., and two 
14-15 mm. By comparing this material with some from 
other localities it appeared that the next year-class, which is 
one year old (group I.), is gathered about a disc-radius of 
3-4 mm. ‘Two-year old individuals (group II.) have a disc- 
radius of 7-8 mm., three-year old ones (groupIII.) 12-1ldmm., 
and four-year old ones (group IV.) 18-20 mm, Pesilaster 
andromeda has attained its maximum of growth at a disc- 
radius of 20-22 mm. It should not therefore attain a greater 
age than four years. The same also holds good for Pontaster 
tenuispinus, Plutonaster bifrons, Bathybiaster veuillifer, and 
probably most of the Northern starfishes, 
Among the species whose age I have determined, Cteno- 
discus crispatus and Hymenaster pellucidus differ from the 
above; the former seems only to attain three years and the 
latter five or possibly six years. c 
27 
