PARALLELISM. 



197 



The'firmisternial structure is a modification of the 

 arciferous, which comes later in the history of growth, 

 and probably also in geological time. During the 

 early stages the Firmisternia have the movable shoul- 



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der girdle which characterizes those of the arciferous 

 division, the consolidation constituting a modification 

 superadded in attaining maturity. Furthermore, young 

 Salientia are toothless, and one section of the species 

 of Arcifera never acquire teeth. In these (the Bu- 



