406 Dr. J. A. Grieg on the Age of some 
there have been living at least three year-classes at the same 
locality. In the cold area off “Tampen” the ‘ Armauer 
Hansen’ in 1914 took three Bathybiaster vemillifer whose 
disc-radius measured 2°7, 3°5, and 25 mm. respectively ; here 
were therefore two quite young individuals which must 
represent one annual class and a_ full-grown individual 
belonging to quite another. 
In the North Atlantic the genus Bathyliaster is represented 
by Bathybiaster robustus, a species which shows so much 
likeness to Bathybiaster vewillifer that Verrill * considered 
it as probably identieal. A closer examination, however, 
shows that they are two different species, each living within 
its own territory—the one within the cold area of the Nor- 
wegian Sea, the other in the warm area of the North Atlantic. 
They also differ from one another in the structure of the 
skeleton, There is the same difference between them that 
there is between Pecten frigidus of the Norwegian Sea and 
Pecten biskayensis of the North Atlantic. §S.W. of Ireland 
the ‘Michael Sars’ obtained 25 specimens of Bathybiaster 
robustus, which are grouped as follows: two specimens 
measured 11-12 mm., the rest 15-21 mm., with a maximum 
of 11 specimens at 19-20 mm. That the maximum is found 
among the largest specimens seems to indicate that the 
smallest (11-12 mm.) specimens must belong to another 
annual class. 
In the eastern part of the North Atlantic Plutonaster 
bifrons is the most commonly distributed starfish. It is 
known from numerous localities between Faroe—Shetland 
Channel and the Cape Verde Islands. In 1910 the 
‘ Michael Sars’ collected this species at no fewer than seven 
localities, at some of them in great numbers—thus at a station 
W. of the Hebrides as many as 87 specimens were found. 
This material has a disc-radius of 8-21 mm., besides which 
there is a specimen of 24 mm. There are two marked 
maxima, one at 10-11 mm. comprising 28 specimens (or 
32:2 per cent. of the total number) and another at 17-18 mm. 
with 17 specimens (or 19°5 per cent. of the total number). 
Both these maxima must represent annual classes. Judging 
from material from another locality the 24 mm. specimen 
must belong to yet another year-class. 
The examples given must suffice. It appears from all the 
material examined that starfishes living at a given locality 
may belong to one year-elass ; but most frequently they 
* Proc. U.S. Nat. Museum, xvii. 1894, p. 256. 
