PLATE XX 



BANGE OF VARIABILITY IN DICTYONEMA FLABELLIFORME AND ITS EVOLUTIONAL 



SIGNIFICANCE 



Varieties of Dictyonema flabelliforme and Callograptua grabaui n. sp. cor- 

 rectly drawn after actual specimens in the collection of Columbia University ; 

 other species and genera sketched after Hall, Ruedemann and Wiman. 



(Reduce all given magnifications by three-fourths.) 



