HAHN, DICTYONEMA-FAUNA OF NAVY ISLAND, N. B. 155 



the organs combined with elasticity and flexibility; if the pull is too 

 great, only concentrated forms may exist. Near the boundary line of the 

 trees of Alpine regions, we find species with slender, movable branches 

 {Betula, Larix), while in places relatively protected from storms, we 

 find dwarf trees and bushes, low and closely clinging {Pinus, Picea) or 

 extremely thickened stems on the slopes of hills exposed to windy squalls. 

 Water-plants lengthen their stalks or build a concentrated mass accord- 

 ing to their standing places. And the same reaction must be true in 

 benthonic animal life. The whole dendrome of Dictyonema was sub- 

 jected to the pull in water, i. e., the mechanical stress exerted against 

 the fixed body by the action of currents, waves and breakers and thus 

 the animal mass proves to be influenced. 

 As a result of this we have : 



I. Var. acadica. This middle type of all the varieties shows very 

 great progress over the small bryograptoid ancestors of Dictyonema, as 

 is proved by the neanastic and nepiastic stages, for the elastic cross- 

 threads enable single branches to grow to such an extent that, for ex- 

 ample, a colony 160 mm. long could have 20,000 theca and yet have 

 sufficient firmness for existence. 



The varieties given here show the complete possibility of improve- 

 ment, but that acadica has not reached the highest place is proved by 

 the fact that of all perfectly preserved examples of later ephebastic 

 stages, acadica has less than 10 per cent, the pyriform paratype up to 

 50 per cent, ruedemanni up to 50 per cent, conferta 70-80 per cent, 

 norwegica less than 10 per cent, desmograptoidea less than 10 per cent. 



Forms like acadica take the middle line of all true Dictyonemas 

 through the Silurian, Devonian into the Carboniferous (compare reti- 

 forme, scalariforme , stenactinotum, spenceri, leroyense, brairi). 



II. Var. norwegica endeavors to progress by a heavy thickening of 

 the rhabdome net, but it causes great weighting down of the body and 

 much profusion of organic matter, so that this mode of building up will 

 be possible only when food is abundant. Compare D. quadrangular e, 

 murrayi, crassum, arayi. 



Eesult : A few sporadic species up to Devonian time; no generic evolu- 

 tion; one-half of one per cent of our forms, more or less broken in 

 pieces. 28 



III. Var. desmograptoidea has closer and even elastic branches join- 

 ing with fewer cross-threads. 



28 Also Westergard mentions the scarcity of unbroken specimens of this variety (4) 

 p. 60. 



