CLASSIFICATION. 



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12. Genus. — Goniodiscus, M. and T. 



Body pentagonal, disciform, flat on both sides. Two rows of large marginal plates at 

 he border, the whole upper surface of which is granulated. The upper as well as the 

 ander marginal plates contribute their share to form the thick border, and divide the 

 same into two equal parts ; the pentagonal form of the disc is therefore maintained by this 

 double row of marginal plates on the flanks and border. The upper and under surfaces 

 are tesselated with a different form of granulation ; vent subcentral. 



13. Genus. — Stellaster, Gray. 



Body nearly pentagonal, flat on both sides, with 

 two rows of large granulated marginal plates, both 

 of which contribute to the formation of the 

 border. Each ventral marginal plate carries a 

 suspended spine. Both surfaces of the disc are 

 covered with granulated plates ; vent subcentral. 



14. Genus. — Asteropsis, M. and T. 



Rays short in proportion to the pentagonal form of the body ; under surface flat ; upper 

 surface more or less elevated ; the rays sometimes ridged. Two rows of marginal plates ; the 

 border sharp, and formed only of one row of plates. The integument on the upper and 

 under surface contains ossicula, but the interspaces of the plates, sometimes also the 

 plates themselves, and always the pore-fields of the upper surface, are completely naked ; 

 vent subcentral. 



15. Ge7ius. — Archaster, M. and T. 



Body flat, with elongated rays. Two rows of large marginal plates ; those of the lower 

 series reach from the under part of the rays to the furrow-plates and are covered with 

 scales, which at the border are metamorphosed into moveable spines ; the upper series 

 are covered with granules, which become bristle-like. The upper surface is level, and 

 closely covered with appendages whose summits are crowned with small bristles. Between 

 the paxillae are single pores ; vent central. 



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