ASTEROTDEA OF THE MERGUI ARCDTPEL A.GO. 



329 



a ray at the base, 15 mm. ; breadth at 10 mm. from the base, 

 10 mm. 



Rays elongate, semicylindrical, flattened actinally, broad and 

 inflated at the base, then rapidly becoming narrower, and main- 

 taining a nearly uniform breadth until near the extremity, which 

 is obtusely rounded. Interbrachial arcs acute. Abactinal area 

 of the disk more or less inflated, marked with well-defined 

 channels along the median interradial lines which extend nearly 

 to the centre. Actinal surface plane ; with the margin abruptly 

 angular. 



The abactinal area is covered with small, narrow, conspicuously 

 crescent-shaped plates, which are disposed in regular alternating 

 longitudinal rows. "Within the concavity is placed a single 

 papula, which is guarded on its adcentral side by small plates 

 (usually two in number, but sometimes more are present), ail 

 included within the arc of the crescentiform plates. The plates 

 bear a great number of uniform microscopic spinelets which form 

 a compact velvet-like covering. The plates along the sides of the 

 ray are somewhat modified in form, often appearing jawbone- 

 shaped rather than crescent-shaped, and the longitudinal dispo- 

 sition of the series is even more conspicuous than in the median 

 radial region. 



The marginal plates are very small, and the infero-marginal 

 series forms the angular ambitus of the ray, the superior series 

 being quite in the lateral wall of the ray. The plates of the 

 inferior series are slightly larger and more definite than their 

 companions, and all bear compact groups of microscopic spinelets 

 similar to those on the abactinal plates. 



The armature of the adambulacral plates consists of : — (1) A 

 furrow series of 7 or 8 spinelets united by membrane, which 

 radiate apart, and form a fan slightly obliquely placed and high 

 in the furrow 7 ; the spinelets are cylindrical, obtusely tipped, and 

 the middle ones are longer than the others. (2) On the actinal 

 surface of the plate is an obliquely placed semicircular fau of 5 or 

 6 obtusely conical, robust, papilliform spinelets which radiate 

 apart ; aud behind these is an irregular tuft of minute cilia-like 

 spinelets. 



In the actinal interradial areas four longitudinal rows of inter- 

 mediate plates extend along the ray up to the extremity ; on the 

 disk four other rows may be counted, but these die out gradually, 

 and do not extend far beyond the base of the ray. The plates of 



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