406 Dr. J. A. Grieg on the Age of some 



there have been living at least three year-classes at the same 

 localit}'. In the cold area off " Tampen " the ' Armauer 

 Hansen ' in 1914 took three Bathyhiaster vexiUifer 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 ^a^Aytiasier is represented 

 by Bathybiafttet' robnstuSy a species which shows so much 

 likeness to B at hy blaster vexiUifer that Verrill * considered 

 it as probably identical. 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 otiier in the w^arm 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 tlie North Atlantic. S.W. of Ireland 

 the ' Michael Sars ' obtained 25 specimens of Bathybiasier 

 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 ram., 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 tlie 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-class ; but most frequently they 



* Proc. U.S. Nat. Museum, xvii. 1894, p. 256. 



