58 FOSSIL ASTEROIDEA. 
depressed border with very minute punctations for the articulation of setze. 
Supero-marginal plates few in number and all subequal in size, forming a broad 
uniform border to the disk. Abactinal area covered with polygonal plates, some 
of which may have stellate or substellate bases, and upon the tabule are borne 
small, more or less co-ordinated granules. Infero-marginal plates more numerous 
than the supero-marginal series, and decreasing in size as they approach the 
extremity ; the surface marked with punctations, which may be co-ordinated in a 
similar manner to that of those on the supero-marginal series, and may be associated 
with granulose elevations. Actinal intermediate plates small, polygonal, covered 
with uniform, crowded, shallow punctations, upon which granules were originally 
borne. Armature of the adambulacral plates arranged in longitudinal lines, which 
may be slightly oblique. Small entrenched pedicellarize may be present occasion- 
ally on the actinal intermediate plates. 
Mitraster is characterised by its cycloid contour, by the equality in size of the 
supero-marginal plates, which do not diminish towards the extremity, and by the 
character of the ornamentation of the marginal plates, especially of the superior 
series. 
The main characters which distinguish this and the two preceding genera are 
distinctly relative, and may be here conveniently compared. In Pentagonaster the 
rays are more or less produced, the supero-marginal plates are more or less 
numerous, and decrease in size as they approach the extremity of the ray, 
and are devoid of a marginal border of sete. In Metopaster the rays are very 
slightly produced, the contour being pentagonal and only slightly extended. 
The supero-marginal plates are few in number, and do not decrease in size as they 
approach the extremity, the ultimate paired plate being larger than the others. 
All are furnished with a marginal border of sete. In Mitraster the contour 
is cycloid almost to the obliteration of the pentagonal form. The supero-marginal 
plates are few in number, but neither decrease nor diminish in size, being subequal 
throughout; and they are furnished with a marginal border of sete. 
I consider that these differences indicate structural characters of sufficient 
morphological significance to render the forms presenting them worthy of recog- 
nition as distinct genera. 
