ASTEROIDEA 153 



border to the abactinal area. Between these 2 rows of enlarged plates are 3 series of 

 smaller ones — that is, the carinal and 2 adradials. 



The position of Mirastrella is not clear. The thin imbricating abactinal plates and a 

 certain similarity to Kampylaster (which in turn is perhaps related to Stegnaster) favour 

 the Asterinidae. The produced inferomarginals and general appearance of the actinal 

 surface suggests the Ganeriidae. 



The species is sexually mature when R measures only 5-5-6 mm. 



Mirastrella biradialis sp.nov. 

 (Fig. D, 4-46 ; Plate IX, fig. i) 



Diagnosis. Rays 5, R 5-5-6 mm., r 3 mm., br 3-5 mm. Form broadly stellate with 

 thick disk, convex abactinal surface, concave actinal surface, and marginal rays sharply 

 defined by the prominent tabula of inferomarginal plates, which are bent obliquely 

 downward and carry 2 or 3 spinelets abruptly larger than abactinal granules; a con- 

 spicuous abactinal interradial furrow and series of oblique superomarginal grooves; 

 granules slightly spaced, uniform ; outlines of plates indicated by differentiated furrows ; 

 adambulacral spines 3 or 4 proximally, usually in oblique transverse series. 



Description. The arched abactinal surface is covered with subequal, slightly spaced, 

 relatively coarse granules or granuliform spinelets invested by a very thin membrane 

 and so disposed that individual plates are indicated by channels between the groups of 

 spinelets. Spinelets spaced about half their own diameter, the contour of the enlarged 

 head of spinelet being round. The spinelets lack an expanded base and measure o-i- 

 0-15 mm. high by 0-09 0-12 mm. broad. Their precise form, for comparison with that 

 of K. mciirvatiis, is best indicated by fig. 4«. The structure of these spinelets is that of a 

 mature animal rather than of a young one. The most characteristic features of the 

 abactinal surface are the furrows (undoubtedly richly ciliated in life) which originate in 

 the bare interval between the prominent inferomarginals, pass obliquely upward between 

 the broad superomarginals, and merge with the channels separating the groups of 

 granules. The interradial sulcus is obviously the largest and is evident after the spinelets 

 have been removed ; its inner end meets the outer border of the primary interradial 

 plate (Fig. D, ^e, x). 



The abactinal plates consist of a small central plate surrounded by 5 primary inter- 

 radials which overlap. Outside of these is an imbricating ring of plates consisting of 5 

 primary radials and 5 pairs of plates external to the interradials. Between the plates of 

 each pair passes the interradial sulcus. These are the innermost plates of the series 

 marked AD' in figs. 4, ^e. Between the primary radial and the terminal plate are 2 

 longitudinal series of imbricating plates occupying the position of the usually single 

 series of radial, or carinal, plates. On either side of this double series (which occupies 

 the crest of ray) are 5 or 6 larger plates composing the first adradial series {ADi). 

 Between these and the superomarginals are 3 plates of a second series {AD 2). Abactinal 

 plates relatively thick. There are no internal processes as in Anseropoda. 



