580 MR. W. PERCY SLADEN ON 
Actinal floor extensive, and occupied by imbricating ventral plates, arranged 
in isolated transverse columns, running from the adambulacral plates to the 
marginal plates ; the whole area covered by a uniform epidermal layer of 
membrane. Each ventral plate bears a single well-developed naked paxilla. 
Adambulacral plates broader than long. Ambulacral spines delicate, taper, 
numerous, irregular in disposition, forming a group which occupies the surface of 
the plate, the size of the spines increasing towards the furrow-margin of the 
plate. 
Mouth-plates forming a pointed mouth-angle, superficies prominent, 
covered with spines similar in form and character to the ambulacral spines, but 
larger. 
Ambulacral sucker-feet with a well-developed fleshy disk, devoid of 
spicules. 
Madreporiform body concealed by paxille. No pedicellarie. 
Mimaster Tizardi, Sladen. 
Mimaster Tizardi, Sladen, 1882, Proc. Roy. Soc. Edin., vol. xi. p. 702. 
Description of an Individual.—F¥orm large and robust ; marginal contour 
stellato-pentagonal. adii five, short and triangular, tapering continuously 
from the base to the extremity, the breadth at the base of a ray greater than 
the lesser radius of the disk, the interbrachial angle being subacute. The 
lesser radius is in the proportion of 45 per cent., R=120 mm., r=54 mm. ; 
R=2-27; breadth of a ray at the base, 58 mm. 
The abactinal surface is high and inflated over the disk, very gibbous at the 
base of the rays but flattening towards the extremities. A deep furrow is 
formed along the median interradial line in consequence of the gibbosity, but 
disappears before reaching the centre of the disk. The actinal surface is more 
or less convex, but in a regular and comparatively slight degree, although the 
feature is probably largely emphasized in the specimen under notice by the 
upward turning of the extremities of the rays, which took place during the rigor 
mortis. Consequent on the curvature of the actinal and abactinal surfaces, 
the margins are very thin and of small dimensions, and are occupied entirely 
by the double series of small marginal plates. The thickness or perpendicular 
height of these two series of marginal plates together is only 4 mm. 
The dorsal area is covered with a great number of small, uniform paxille, 
closely and equidistantly placed, and with a well-defined space between each, 
and presenting no definite order of arrangement, excepting in the immediate 
neighbourhood of the arm-angle, where a certain amount of obliquely transverse © 
lineal disposition may be observed. The whole of the calcareous portion of the 
