NIAGARA GROTTP. 



237 



ECHINID^. 



[Carboniferous] Arckceocidaris. 

 [Carboniferous] Palachinus. 



[Lower Silurian] Agelacrinites. 



[Lower Silurian] Sphsronites 



AstekiaDjE. Ophiueid^. 



Pentremites [Devonian and Car- 

 I boniferous]. 



Apiocystites [Upper Silurian]. 



Pseudocrinites [Upper Silurian]. 



Prunocystites [Upper Silurian]. 



Echino-encrinites [Upper Silurian]. 



Cystide^. 

 [Lower Silurian] Caryocystites. Cryptocrinites [Lower Silurian]. 



[Lower Silurian]. 



Hemicosmites. 



Caryocrinus. 



Crinoide^. 



[Lower Silurian]. 

 [Upper Silurian]. 



In this diagram we begin at the base with Crinoide^, and through Caryocrinus very naturally 

 pass to Hemicosmites, a true cystidean. The word Cystide^ occupies the centre, around which 

 are arranged the genera of true Cystideans. On the one hand we pass from Hemicosmites, 

 through Caryocystites, SphcBronites, Agelacrinites, all true Cystideans, by the genera Palce- 

 chinus and Arckceocidaris, which link the former with the other great sections of Echinodermata. 

 On the other hand, we pass from Hemicosmites, by Cryptocrinites, Echino-encrinites, Pruno- 

 cystites, Pseudocrinites, Apiocystites, all true Cystideans, by the genus Pentremites, to the 

 AsTERiADiE, and by divergence to OpHiURiDiE. 



Let us examine for one moment the direction of these lines of progressive development in 

 the geological series, and see whether there are not some objections to this view, or at least 

 some important links wanting in the chronological order. Caryocrinus is an upper or middle 

 Silurian fossil : Hemicosmites is lower silurian ; and, in point of time, we go backward from 

 the former to the latter. Starting with Hemicosmites, we pass, on the one hand, through the three 

 genera of Cystideans, all of which are of lower silurian age, and link them with Echinidae by 



