240 MINERALS AFD GEOLOGY 



2. arms without pinnulce. 



Palceocrinus : arms long and thin, of equal size, without pinnulse, 

 bifurcating. Lower Silurian. 



Hybocrinus : arms very long and thin, not bifurcating, and with- 

 out pinnulse ; Lower Silurian. 



Cheirocrinus (including Calcecrinus, &c.): arms very long, decum- 

 bent, unequal in size, with single row of plates. Silurian, Devonian. 



Ichthyocrimts : arms short, without pinnulse, more or less in 

 close contact throughout their length, gradually 

 merging into the calyx ; arm plates in one row ; 

 stem round, with small circular orifice. Roof of 

 calyx composed of small, imbricating plates. Upper 

 Silurian to Carboniferous. Figure 155 represents 

 our principle species. 



Lecanocrinus : Closely allied to Ichthyocrinus 

 (if not really identical), but calyx plates larger and 

 less numerous. L. elegans, Trenton Formation, is 

 our best known species. 



r IG. 155. 



Other typical genera of the tesselated crinoids, Ichthyocrinus icevis. 

 comprise : Pisocrinus, Marsupites (a stemless form), Nia ^ ara Formati011 - 

 Actinocrinus, Crotalocrinus, <fcc. The Articulata (see above) comprise, 

 more especially, Enerinus (to which the " lily enerinite," E. liliifor- 

 mis, of Triassic strata, belongs) ; Apiocrinus (including the " pear 

 enerinite " of Jurassic strata); Pentacrinus, Antedon or Comatula, 

 &c, but no examples of this group occur in Central Canada. 



II. 



CYSTIDEA. 



The Cystideans from an entirely extinct group of Cambrian and 

 Silurian age. They present relations, in some cases obscure, in 

 others very marked, to both crinoids and blastoids. The typical 

 cystidean may be described as an oval or spherical body, covered 



two distinctive sections, the groups based on the presence or absence of a canaliculated struc- 

 ture in the plates of the calyx and arms. A synopsis is published in the Transactions of the 

 Royal Society of Canada : vol. 1, pp. 113-116. This grouping is not followed here as in a little 

 work of this unpretending character (dealing only with local forms, necessarily limited in 

 number) it would be out of place to enter into the classification details which it involves 



