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OOLITIC GONIASTERID^. 



Additional Genera of the Family Goniasterid^. 



ASTER, Gray, 1841. 



Body nearly pentagonal, flat on both sides, with two wide rows of large granulated 

 marginal plates, both of which contribute to the formation of the high border. Each 

 ventral marginal plate carries a suspended spine. Both surfaces of the disk are covered 

 with granulated plates ; vent subcentral. The ambulacra are narrow and the suckers 

 biserial. 



The genus Goniaster was proposed by Professor Agassiz, in his ' Prodrome,' to include 

 Starfishes with a large pentagonal body, having the margin bordered by a series of wide 

 thick plates or tesserse, superimposed in pairs, and which support spines, granules, &c. 

 The upper and under surface of the disk is covered with small polygonal plates, set closely 

 together like a mosaic, and fitted into this marginal framework ; the surface of the 

 ossicles is smooth or covered with granulations. The ambulacral furrows are narrow, 

 wdth two rows of pedal suckers therein ; the vent opens near the dorsal surface, and the 

 mouth-opening is slit-like and pentagonal. 



Miiller and Troschel, in their 'System der Asteriden,' suppressed the genus Goniaster, 

 and instead thereof erected the genus Astrocjonium, adding also their new genus Gonio- 

 discus and Gray's Stellaster. 



The diagnostic characters of these groups were chiefly obtained from the structure 

 of the marginal plates and their appendages, the figure and arrangement of the discal 

 plates, and the form and development of the rays, and may be thus described : 



1. Astrogonium. — Marginal plates large and smooth towards the centre, their inner 

 border encircled by granules. 



2. Goniodiscm. — Marginal plates having the entire upper surface covered with close- 

 set granulations. 



3. Stellaster. — Marginal plates granulated, the ventral segment supporting a pendant 

 spine, the rays elongated and tapering to a lanceolate extremity. 



Many of these characters are absent in fossil Goniaster idee, and are, therefore, value- 

 less for palgeontological purposes ; for this reason I have retained — 



(a) The genus Goniaster for the large pentagonal short-rayed forms, and 



(b) The genus Stellaster for those with a smaller disk and more elongated rays. 

 This division must be considered merely provisional until we become better 



acquainted with the comparative anatomy of extinct forms. 



The very fine fossil discovered by my friend Samuel Sharp, Esq., F.G.S., in the 



