MIMASTER, A NEW GENUS OF ASTEROIDEA. 581 
abactinal skeleton is composed entirely of paxillee, as in Astropectinide. The 
paxillz consist of a cylindrical pedicel, about twice as high as broad, expanding 
slightly at the base, and with the distal extremity rounded and clavate, and 
surmounted by a crown of 15 to 20 spinelets, which radiate apart very slightly 
and produce a compact form of paxilla. The spinelets are short, delicate, and 
slightly taper, about equal in length to the pedicel, and sometimes less, 
probably owing, to a certain extent, to abrasion. The base of the paxilla is 
quite small and thin at the margin, where a faint tendency to develop rudi- 
ments of two or three very short radiating processes may be noticed. No 
calcareous union or connection exists between individual paxille. Numerous 
small papule occur in the interspaces, three to five being present in the quad- 
rangle formed by four neighbouring paxille. Their membrane is very delicate, 
and they taper somewhat rapidly towards the tip, which is thickened intoa 
small knob. Owing to the manner in which the papule taper, a comparatively 
swollen appearance is given to their lower part. 
The marginal plates are small and subtubercular in appearance, and are 
arranged in ventral and dorsal series, 37 to 38 plates being present in 
each, between the interbrachial angle and the extremity of the ray. Each 
plate is rounded or boss-like externally, and covered with a great number 
of small spinelets similar to those of the paxillee, which gives them a prominent, 
cushion-like appearance. The infero-marginal (or ventro-marginal) plates are 
the largest, transversely sub-oval in form—the length increasing towards the 
interbrachial angle—and bear not less than 100 spinelets. The supero- 
marginal (or dorso-marginal) plates are smaller, usually round, and are placed 
rather more aborally than the companion plate of the lower series, the pairs 
standing consequently slightly oblique. 
The ventral plates occupy a great space on the actinal surface, and extend 
up to the very extremity of the ray. The plates are oblong and are arranged 
in regular transverse and slightly oblique lines between the adambulacral 
plates and the marginal plates. Each series or column thus formed is isolated, 
being separated from the neighbouring column by a narrow space; and each 
plate in a column overlaps or imbricates upon the next innermost plate. The 
number of the columns corresponds exactly with that of the adambulacral 
plates, and is not in relation with that of the marginal plates. The whole 
ventral area is overlaid by a uniform layer of membrane, by which the shape of 
the individual ventral plates is hidden from superficial observation. Each 
ventral plate bears a single paxilla near its free extremity, which is rather more 
robust than those on the dorsal surface, and carries rather fewer spinelets, 
which are somewhat longer and more widely expanded. The paxillee, like 
those on the dorsal area, are naked and not invested in membrane. In 
