PALEOZOIC FOSSILS. 301 



Now the specimen of this species which I have examined, and which I 

 suppose to be the one figured upon Plate x, figure 3 a, ^, has the ambula- 

 cral plates alternating; and however these minute structures may present 

 themselves to our eye, I believe that we have in no Echinoderm two adja- 

 cent series of plates which are precisely opposite one another. 



In the specimen of T. cylindricus examined, the ambulacral plates are 

 less distinctly alternate ; but the relation of the adambulacral plates and 

 the pores are the same as in the other form. Not having seen the specimen 

 showing the dorsal view, figure 4 a of Plate x, I can only remark that the 

 structure of the rays is very similar to that of Protaster. If the 

 appearance of a disc be fallacious, then we have in T^niaster a structure 

 in all respects similar to that of Protaster, wanting the disc. If the 

 structure of Protaster, as represented by Mr. Salter, be the true one, 

 then the New-York species must be referred to another genus. Notwith- 

 standing the differences shown between the figures of Prof. Forbes and 

 Mr. Salter, and between these and the illustrations here given, I am still 

 inclined to believe that our species is congeneric with the original of Prof. 

 Forbes's type of that genus. 



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