Arctic and yorllt-Allantic Starfishes. 407 



rrpiesent two or more, only oiio of them being nimierous. 

 The material also slious that each yea i -class is grouped ahout 

 certain definite sizes, and tiiat these as a whole are cominou 

 to all s|)ecies examined. I'ontdster lenuispiuus has a year- 

 class Avhose maximum is gathered ahout a disc-radius of 

 3-4 mm. ; Cie/iO'/iscus crisputuSy Leptychaster arciicuSy 

 Biitfnjhioster vexilltfer, >Solaster papposus, Ili/menaster pellu- 

 ctJiiSj Ilenri'cia samjuinolenta, etc., have likewise a year-class 

 of this size ; Pontasler tenuispinus has the next year-class at 

 8-10 mm. In like mannor we find that Plutonaster hifronSy 

 D(ftaster agassizi, Ctenodiscus crispatus, IJijmenaster pellu- 

 cidnSj 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. 

 Sfieaking generally, these are agreements which cannot be 

 due to accidental circumstances. Each of these year-classes 

 must have a fixed age, and if the year-class 3-4 mm. of 

 Pontasler 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 specie?, 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 nitn., three 7-8 mm., four 10-12 n)m., 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 nun. Two-year old individuals (group II.) have a disc- 

 radius of 7-8 mm., three-year old ones (groupIII.) 12-15 mm., 

 and tour-year old ones (group IV.) 18-20 mm. Psilaster 

 andromeda has attained its maximum of growth at a disc- 

 radiua of 20-22 mm. It should not therefore attain a greater 

 age than four years. The same also holds good for Pontasler 

 tenuispinus^ Plutonaster lifrons, Bathijbiasler vexillifer, and 

 j)robably most of the Northern starfishes. 



Among the species whose age 1 have determined, Cteno- 

 discus crispatus and iJynienasler pellucidus differ from the 

 above ; the tbrmcr seems only to attain three years and the 

 latter five or possibly six years. 



27* 



