THE ORDOVICIAN PERIOD. 



363 



much or more due to these as to unfavorable physical conditions. 

 Rather paradoxically it may perhaps be said that the very scantiness 

 of life relics is due to the abundance of certain forms of life. 



Fig. 168. — Ordovician Sponges, a, Receptaculites occidentalis Salter; b, Brachio- 

 spongia digitata Beecher; c, Archceocyathus minganensis Billings; d, Strot - 

 spongia maculosa U. and E. : e, Ischadites, species undetermined. The Recep- 

 taculites and Ischadites have sometimes been regarded as giant foraminifers. 



Ecological, social, and mental development. — It is not necessary 

 to more than briefly recur to the ecological, social, and psychological 

 considerations to which attention was called in the discussion of the 

 Cambrian life, for they remain essentially the same, save that the ampler 



e 



Fig. 169. — Ordovician Annelids, a, Arabellites 

 catus Hinde; c, Eunicites gracilis Hinde; d 

 cites varians (Grinnell) ; /, Oenonites rostratus 



Figures a, b, c, d, e, and / are highly magnified illustrations of the jaws of annelids 

 Figure g is an example of the calcareous tubes secreted by some annelids attached 

 to other shells. 



cornuius Hinde; b, Glycerites sul- 

 Arabellites ovalis Hinde; e, Euni- 

 Hinde; g, Ortonia minor Nicholson. 



development and better preservation of the Ordovician fauna give 

 greater definiteness to the problems and supply more data for their 

 elucidation. It seems clear that the adaptation of the various forms of 

 life to one another and to their physical environment had reached a 

 higher stage of adjustment, indeed a degree of adjustment not very 

 greatly inferior to that which now prevails among the corresponding 



