16 



JAMES A. GRIEG 



[REP. OF THE "MICHAEL SARS" NORTH 



specten irregularis has not since been found again in those 

 northern waters. Mr. Dons, keeper of the Tromsoe Mu- 

 seum, has thus kindly informed me that there is no 

 specimen of Astrospecten irregularis in the Museum. I 

 have not found it myself among the rich echinoderm 

 material brought home from northern Norway and Barents 

 Sea by Mr. Nordgaard and by the "Michael Sars". 



Bathybiaster robustus Verrill. 

 Archaster robustus Verrill, Amer. Journ., ser. 3 vol.28, 1884, p. 383. 



21 h. Stat. 95, 50° 22' N, 11° 44' W, 1797, temp. 3,5° Cel. 25 

 specimens. 



6 /s— Vs. Stat. 101, 57°41'N, ir48'W, 1853, temp. 3.3° Cel. 30 

 specimens. 



The dimensions of the largest specimens from stat. 

 101 were: — Arm-radius 127 mm., disc-radius 23 mm., 

 breath of arm at base 26 mm., 57 dorso-marginal plates. 

 In the other specimen these figures are 80 mm., 17 mm., 

 18 mm. respectively and 53 dorso-marginal plates and in 

 the third one 56 mm., 14 mm., 15 mm., 44 dorso-marginal 

 plates. The proportions of breadth of arm to arm-radius 

 is 1 : 4.88, 1 : 4.44 and 1 : 3.39 respectively in these spec- 

 imens. The proportion r : R is 1 : 5.52, 1 : 4.72 and 1 : 4. 

 Young individuals have proportionately broader and shorter 

 arms than older ones, as will be even more evident from 

 following table of measurements of some specimens from 

 stat. 95. 



Arm- 

 radius 



Disc- 

 radius 



Breadth of 

 arm at 

 base. 



r 



:R 



A 



:R 



Number of 



dorso- 

 marginal 

 plates. 



53 



» 



14 



1 



4.42 



1 : 



3.79 



44 



54 



11 



15 



1 



4.91 



1 : 



3.60 



46 



68 



15 



16 



1 



4.53 



1 : 



4.25 



49 



76 



15 



18 



1 



5.07 



1 : 



4.22 



50 



78 



16 



19 



1 



4.87 



1 : 



4.11 



53 



83 



20 



23 



1 



4.15 



1 : 



3.61 



43 



89 



17 



18 



1 



5.24 



1 : 



4.91 



52 



92 



18 



20 



1 



5.11 



1 : 



4.60 



58 



94 



18 



22 



1 



5.22 



1 : 



4.27 



57 



97 



17 



19 



1 



5.71 



1 ; 



5.ii 



62 



100 



19 



23 



1 



5.26 



1 : 



4.35 



58 



102 



19 



20 



1 



5.36 



1 : 



5.oi 



58 



The measurements are in millimetres. 



The table further shows that the proportion of r : R 

 and of length of the arms to their breadth are subject 

 to great individual variations. Thus the comparatively 

 shortest length of arm is not found in the smallest spec- 

 imen in the table but in one of middle size. Even in 

 specimens of the same size these figures may vary. Thus 



in three specimens with an arm-radius of 99 mm. the 

 proportion r : R is 1 : 5. 21, 1 : 5.5 and 1 : 5.82 respectively, 

 and A : R is 1 : 4. 71, 1 : 4.75 and 1 : 5.50. 



The marginal plates are high and narrow. Apart 

 from the innermost interbrachial plates the proximal plates 

 are largest, gradually decreasing in size towards the points 

 of the arms. In the largest specimen the measurements 

 were : 



1st dorso-marginal plate 1.5 mm. long 7.5 mm. high 

 10th -„- 2.5 - „ 7 - 



20th -„- 2.5 - „ 6.5 - 



30th -,- 2 - „ 4.5 - 



40th -„— 2 - „ 3 - 



50th -„— 1 „ 1.5 - 



At the tipper edge of the dorso-marginal plates one 

 or, very rarely, two conical knobs were situated, which 

 may, however, be absent in young individuals. Larger 

 specimens have often besides these a somewhat smaller 

 knob in the centre of the plate. In younger specimens 

 one or two knobs are situated along the distal margin 

 of the ventro-marginal plates of the arm-angle. In larger 

 specimens there are 3 or, more rarely, 4 knobs. The 

 uppermost of these is situated a little below the edge 

 nearest the dorso-marginal plates, the lowermost near the 

 lower edge of the plate, and the third, about the middle 

 of it. They decrease in number towards the points of the 

 arms, the outermost plate bearing only one knob. The knobs 

 of the ventro-marginal plates are smaller than those of 

 the dorso-marginal ones. They may, however, be absent 

 in the former as well as in the latter. 



The central "epiproctal cone" is large and well- 

 developed in young individuals, but gradually disappears 

 with age. It may, however, also be present in older 

 individuals and was thus very distinct in a specimen 

 whose arm-radius war 92 mm. On the other hand it was 

 absent in one with arm-radius 78 mm. 



The abactinal skeleton is composed of round or 

 irregularly polygonal plates which join, but never over- 

 lap Owing to the irregular shape of the plates small 

 open spaces are formed between them in which 

 a papula is placed. Each plate bears a little low cylin- 

 drical paxilla whose upper surface is furnished with 

 as many as 12 granules. In the closely related Bathybi- 

 aster vexillifer the skeleton is composed of stellate over- 

 lapping calcareous plates, each bearing a paxilla similar 

 in shape and appearance to that of Bathybiaster robustus 

 but apparently a little more slender than in that species. 

 The abactinal skeleton therefore affords a good specific 

 character for these two closely related and in their habits 

 so similar species. 



The actinal skeleton is covered with scales. In some 

 individuals several of the scales are converted into small 



