ATLANT. DEEP-SEA EXPED. 1910. VOL. III.] 



ECH1NODERMATA 



35 



one example each. The smallest of these specimens must 

 belong to a different year-class from that of the largest, 

 but whether representatives of a third year-class are found 

 among them it is impossible to decide, as the material 

 is very scanty. Ophiocten sericeum, which was so 

 numerous at stat. 70 seems to have been represented 

 there by a single year-class, and as all of the individuals 

 measured 2 to 5 mm., they were probably one year old. 

 33 examples of Ophiacantha abyssicola were obtained at 

 stat. 101 (disc-diameter 4—8 mm.) These specimens may 

 be grouped as follows: 



4 (1), 5 (4), 6 (6), 7 (4) and 8 (8) 

 (The figures in brackets represent the number of 

 specimens; the others the size of the disc-diameter in 

 millimetres.) 



The specimens from stat. 101 are grouped about a 

 disc-diameter of 7 mm. thus probably also ^present a 

 single year-class. 



If we were to draw any conclusion from this somewhat 

 scanty material, it must be, that what has been said in 

 the foregoing about Ophiomusium lymani applies 

 equally to all of the deep-sea ophiurids of the North 

 Atlantic. 



For the purpose of comparison I shall give in the 

 following table a summary of the grouping of sizes in 

 some of the North Atlantic deep-sea star-fishes, of which 

 more abundant material is at my disposal. In determining 

 sizes I have found it most convenient to use the radius 

 of the disc for the star-fishes and not the disc-diameter 

 as in ophiurids. 



As the table shows there is a great abundance of 

 specimens of Plutonaster bifrons, particularly from stat. 

 101. We find a group of 96 individuals from this lo- 

 cality with a disc-radius of 8 to 21 mm., and one 

 specimen at 24 mm. The group affords two maxima; 

 one consisting of 28 individuals at 10 — 11 mm. and the 

 other of 17 at 17 — 18 mm. Both of these must repre- 

 sent a year-class. It is not possible to determine defi- 

 nitely whether the 24 mm. specimens belong to the older 

 of these year-classes or whether it represents a third one, 

 but judging by the material from stat. 41 I am most 

 inclined to think that stat. 101 had 3 year-classes. At 

 stat. 95 there is a large group of 16 individuals with a 

 disc-radius of 14 — 20 mm. and one 8 mm. specimen, which 

 must belong to a year-class different from the large group. 

 If we compare these specimens with those from stat. 101, 

 it is evident that the large group must be of the same 

 age as the group from stat. 101 which is grouped about 

 a disc-radius of 17—18 mm., while the 8 mm. example 

 must belong to the youngest year-class. At stat. 41 there 

 were three year-classes. The material comprised 14 speci- 



mens, one of which measured 10 mm., another 27 mm., 

 the remainder 14—22 mm. The latter must belong to 

 the same year-class as the large group at stat. 95, while 

 the 10 mm. specimen must be of the same age as the 

 8 mm. example in that locality. The 27 mm. specimen 

 must belong to a third and older year-class. The mate- 

 rial from stat. 24 is very little differentiated. It is com- 

 posed of two small groups, one of 9 individuals with a 

 disc-radius of 13 — 18 mm, the other of 6 individuals with 

 a disc-radius of 18 — 20 mm. If we compare these groups 

 with the material from the other localities it seems prob- 

 able, that there were two year-classes at stat. 24. 



The examples of Dy taster agassizi from stat. 88 

 must all belong to the same year-class, as they have a 

 disc-radius of 6 — 12 mm. The 9 — 10 mm. group affords 

 a maximum of 18 individuals or 53 per cent of the total 

 number. The specimens of Benthopecten spinosus from 

 stat. 101 must probably likewise represent a year-class. 

 The features of the material were insufficiently marked, 

 however. 



Disc- 

 radius 



Bentho- 

 pecten 

 spinosus 



Plutonaster bifrons 



Dytaster 

 agassizi 



■K £ « 

 2 -o U 

 £- - = 



>!_^ -d 

 3? 2 £ 



22 p 



ol.it. 



oiat. 



Stat. 



Stat. 



Stat. 



Stat. 



Stat. 



Stat. 



in mm. 



101 



24 



41 



95 



101 



88 



101 



101 









Number 



of specimens 







5 



1 

















6 



2 











2 



3 





7 



2 











3 



1 





8 



3 







1 



1 



4 



3 





9 



2 









5 



8 







10 



2 





1 





12 



10 



2 





11 













16 



4 





1 



12 











7 



3 





1 



13 





1 







7 









14 





4 



1 



2 



4 





1 





15 





4 



1 



2 



4 





1 



3 



16 







2 



5 



6 





1 



2 



17 







2 



4 



8 





1 



2 



18 





1 



2 



2 



9 





2 



2 



19 





3 



2 





4 





2 



6 



20 





2 





I 



2 





3 



5 



21 







1 





1 





1 



3 



22 







1 









2 





23 

 24 



25 











1 























26 

 27 







1 





— 











12 



15 



14 



17 



97 



34 



23 



25 



