REPORT ON THE ASTEROIDEA. 



333 



Chorologiccd Synopsis of the Species. 



1. Mimaster tizardi, Sladen. 



Mimaster Tizardi, Sladen, 1882, Proc. Roy. Soc. Edin., vol. xi. p. 702; Trans. Roy. Soc. Edin., vol. 

 xxx., part ii., p. 580, pi. xxxiv. 



Rays five. R=120 mm. ; r=54 mm. R = 2 - 2 r. The minor radius is thus in the. 

 proportion of 45 per cent. Breadth of a ray at the base, 58 mm. 



General form large and robust. Marginal contour stellato-pentagonal. Rays short 

 and triangular, tapering continuously from the base to the extremity, the breadth at the 

 base of a ray greater than the minor radius of the disk, the interbrachial arc being 

 subacute. 



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 to a comparatively 

 slight degree, although the feature is probably largely emphasized by the upward turning 

 of the extremities of the rays in consequence of their posture at the time of death. 

 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 the two series of marginal 

 plates together is only 4 mm. 



The abactinal surface is covered with a great number of small uniform paxillas, closely 

 and equidistantly placed, and with a well-defined space between each, which present 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 abactinal skeleton is composed entirely of 

 paxillse, as in the Astropectinidoe. The paxilla? consist of a cylindrical pedicle, 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 fifteen to twenty spinelcts, which radiate apart 

 very slightly and produce a compact form of paxillae. The spinelets are short, delicate, 

 and slightly taper, about equal in length to the pedicle, and sometimes less, probably 

 owing to a certain extent to abrasion. The base of the paxillse is quite small and thin 



