58 FOSSIL ASTEROIDEA. 



depressed border with very minute punctations for the articulation of setae. 

 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 tabulae 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 pedicellarise 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-mai-ginal 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 setae. In Meto-paster 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 setse. 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 setae. 



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. 



