NIAGARA GROUP. 
237 
Echinid.®. 
[Carboniferous] Archoeocidaris. 
[Carboniferous] Palcechinus. 
[Lower Silurian] Agelacrinites. 
[Lower Silurian] Sphaeronites. 
Asteriad.®, Ophiurid®. 
Pentremites [Devonian and Car- 
j boniferous]. 
Apiocystites [Upper Silurian]. 
Pseudocrinites [Upper Silurian]. 
Prunocystites [Upper Silurian]. 
Echino-encrinites [Upper Silurian]. 
Cryptocrinites [Lower Silurian]. 
[Lower Silurian]. 
CYSTIDE®. 
[Lower Silurian] Caryocystites. 
[Lower Silurian]. Hemicosmites. 
Caryocrinus. [Upper Silurian]. 
CrINOIDE®. 
In this diagram we begin at the base with Crinoideje, 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, Sphaeronites , Agelacrinites , all true Cystideans, by the genera Palce¬ 
chinus and Arcliceocidaris , 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 
AsTEMADiE, and by divergence to Ophiurid®. 
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 
