REPORT ON THE ASTEROIDEA. 83 



II. Madreporiform body simple, exposed. Adambulacral armature all spinifonn, in co- 

 ordinated groups, subastropectenoid. Adambulacral plates broad. (Tdhyuster.) 



A. Rays long. Marginal plates seventy to eighty in the adult .... subinermis. 



B. Rays short. Marginal plates twenty to thirty in the adult .... parelii. 



Chorology of the Genus Plutonaster. 



a. Geographical distribution : — 



Atlantic : Five species between the parallels of 75° N. and 10° S. 



Plutonaster rigidus and Plutonaster rigidus, var. semiarmata, off 

 the east coast of the United States of America. Plutonaster abbre- 

 viatus and Plutonaster notatus, in the neighbourhood of the Azores. 

 Plutonaster bifrons in the Faeroe Channel, off the coast of Portugal, 

 and off the coast of North America (the latter occurrence on the 

 evidence of a single immature specimen, which might belong to a 

 variety). This species has also been obtained (Jide Norman 1 ) in the 

 Barents Sea between Spitzbergen and Nova Zembla, in lat. 73° 41' 

 12" N., long. 22° 58' 30" E., at a depth of 210 'fathoms. The 

 examples are stated to be young. Plutonaster marginatus in the 

 South Atlantic, near the Island of Ascension. 



Pacific : One species between the parallels of 30° and 40° S. 



Plutonaster ambiguus, off the east coast of Australia, near 

 Sydney. 



/3. Bathymetrical range: 210 fathoms to 1360 or 1700 2 fathoms. 



All the species except Plutonaster marginatus belong exclusively to the 

 abyssal zone. Of the remaining five, two only, Plutonaster bifrons and Pluto- 

 naster ambiguus, have been found in depths less than 1000 fathoms. 



Greatest range of one species : Plutonaster bifrons, 210 to 1360 fathoms. 



y. Nature of the Sea-bottom : Two species, Plutonaster rigidus and Plutonaster 

 bifrons, are found on the Blue mud, and Plutonaster bifrons is also found on 

 Globigerina ooze. Plutonaster ambiguus is found on Green mud. Plutonaster 

 notatus is found on Pteropod ooze ; the nearly allied Plutonaster abbreviatus 

 on Volcanic mud. Plutonaster marginatus on Volcanic sand. 



1 In W. S. M. D'TJrban on the Zoology of Barents Sea, Ann. and Mag. Nat. Hist., ser. 5, vol. vi., 1880, 

 p. 260. 



2 The uncertainty in the limit of greatest depth arises from not knowing at which of two stations Plutonaster 

 rigidus was dredged. 



