282 GEOLOGICAL BIOLOGY. 



Significance of the Facts. — By turning back to the table 

 representing the geological range of the several genera and 

 families of the Helicopegmata (p. 280) it will be seen that 

 the total life-range of all the representatives of the group 

 extends over eleven periods of the time-scale (from the 

 Neo-ordovician to the Jurassic). In the Neo-ordovician there 

 appeared a fevv^ small representatives of one of the families, 

 but in the next period (Eosilurian) all three of the families are 

 represented. In other words, all of the family differentiation 

 was attained in, we may say, the first decade of the life of the 

 suborder, and there were in the Silurian 5 genera of Atrypi- 

 dae, 5 genera of Spiriferidse, and 1 1 genera of Athyridae. 



All the essential extrinsic characters of the brachidium 

 which ever appeared had arisen at the very outset or initial 

 stage of the history of the group of organisms possessing the 

 brachidium. 



When we consider that in evolution the real increment 

 in any case is seen in the acquirement of differences in the 

 morphological characters of organisms, and it is not a new 

 species or genus or order that is evolved, but it is the develop- 

 ment by individuals of some part of their organization in a 

 different form from that seen among their ancestors, the sig- 

 nificance of this observation is apparent. 



After this initial stage there are no representatives of the 

 whole order Helicopegmata in which the relative position of 

 the loop is not found to be of generic value in taxonomic 

 classification, and there is no case in which the modification 

 of this character surpasses the limits attained at this initial 

 stage of evolution. 



The Loop of the Ancylobrachia and the Brachidium of Heli- 

 copegmata. — This was in all probability near the time of 

 divergence of the Ancylobrachia and Helicopegmata, and as 

 has been suggested,* the fundamental difference between 

 the calcified brachial supports of these important groups of 



to Fischer's "Manuel de Conchyliologie " on " Brachiopodes ;" to Zittel's 

 " Handbuch der Palaeontologie," vol. i., and to Davidson's classic treatise on the 

 "British Fossil Brachiopoda." 



* "On the Brachial Apparatus of Hinged Brachiopoda and on their Phy- 

 logeny," Proc. Rochester Acad. Sci., vol. II. p. 113, etc., 1893. 



