152 DISCOVERY REPORTS 



spines. The abactinal granules are uniformly distributed and no bare furrows proceeding 

 inwards from the interval between marginal plates are present, as in M. hiradialis. 



Figures of the spinelets of these young (figs. 3 Z>, 3 c) are of significance for comparison 

 with those of the adult. 



Koehler, by comparing Kampylaster with Tremaster Verrill (Fisher, 191 1, p. 254), 

 implies a certain similarity between the two genera which is rather far-fetched. Tremaster 

 is peculiar in having 5 interradial perforations similar to those of " key-hole " sand dollars 

 (e.g. Mellita). Stegnaster Sladen seems to be a much nearer relative and one which also 

 has a habit of folding up when killed in alcohol (Mortensen, 1925, p. 303, pi. 13, 

 fig. II). 



Type locality. Ofl^ Adelie Land, 66° 32' S, 141° 39' E, 151 fathoms. 



Distribution. Probably circumpolar: 97° 20' £-141° 39' E; Ross Sea; Palmer 

 Archipelago; bathymetric range 93-351 m. 



Mirastrella gen.nov. 



Diagnosis. Similar to Kampylaster but diff'ering in having a double series of equiva- 

 lent abactinal radial plates, tabulate inferomarginal plates, and specialized abactinal 

 interradial grooves. Type M. biradialis. 



Although there is much similarity in general appearance between Mirastrella and 

 Kampylaster, the latter does not have the very unusual double row of carinal or radial 

 plates. The plates of its normal single series are separated by encroachment of the 

 adradials of either side which imbricate and force the radial plates apart. As the animal 

 grows, irregularity of plates often obscures the radial series. 



The young of K. incurvatus , recovered from the brood chamber, much more nearly 

 resemble Mirastrella than do the adults. In these young the inferomarginals (4 to a ray) 

 constitute a " phanerozoniate " border but there is no specialized tabulum. On the 

 ventrolateral margin of plates are 2 or 3 prominent spinelets which stand out laterally 

 and form an ambital fringe. Above these on the dorsal surface ef plate are 3-5 smaller 

 spinelets a trifle larger than the superomarginals and abactinals. As mentioned under 

 incurvatus the young " grow up to " the ventrolateral spinelets which are not enlarged in 

 the adult. 



Mirastrella is readily distinguishable from K. incurvatus by the presence of this 

 marginal fringe of spinelets, the tabulate character of the inferomarginal plates, and by 

 the abactinal grooves. The spinelets of Mirastrella lack the subcircular, perforated, 

 basal expansion found in Kamplylaster while the membranous investment of the body is 

 much thinner in Mirastrella. 



Mirastrella superficially resembles Leilaster A. H. Clark (1938, p. i, pi. i), the type of 

 which is Korethraster radians Perrier from off Barbados, 56 fathoms. I have examined a 

 specimen of Leilaster which appears to belong to the Ganeriidae. Its produced infero- 

 marginals are similar to those of Mirastrella but the abactinal skeleton is very different. 

 There is a normal carinal series of plates and on either side a series of subequal adradials. 

 The adradials of the second series, however, are conspicuously enlarged and form a 



