66 NEW YORK STATE MUSEUM 



geosyncline in the east, and in the great geosyncline of the Rock)' mountain 

 region in the west (Nevada, British Columbia) and connecting the two, the 

 occurrences in Arkansas and Indian Territory, which may well be considered 

 as the northern edge of a long curved depression that in the south con- 

 nected the Appalachian geosyncline with the great Rock)- mountain geosyn- 

 cline without, however, leading there to like extensive folding. This great 

 semicircular depression bounded the vast platform that stretched southward 

 from the Canadian shield and was flooded in Champlainic time by a 

 shallower epicontinental sea. 



Synoptic view of the range of the graptolite genera of the United States 1 

 The synoptic table following serves (i) to illustrate most concisely the 

 hemeras and the relative vitality of the orders and genera of our graptolites, 

 (2) to suggest phylogenetic connections and (3) to show more distinctly 

 the relations of the zones than long verbal explanations could do. We will 

 for these reasons consider the ranges of the genera separately from the 

 discussions of the zones and briefly note the points illustrated by the table. 

 In surveying the relative length of the biotic lines — which by the 

 way are here made to begin and terminate bluntly at the zonal boundaries, 

 principally to hide our ignorance as to the actual points of origination and 

 termination of the genera within the zones — one will at once notice the 

 disproportionate lengths of the biotic lines of several of the Dendroidea. 

 Foremost of these is Dictyonema which extends from the Cambric to the 

 Carbonic and apparently possesses two culminating periods (according to 

 number of species), one in the early Champlainic and one in the Niagaran- 

 Lower Devonic time. It is to be assumed that the appearance of these 

 two culminating periods is a deception due to our as yet incomplete knowl- 

 edge of the Dendroid faunas. It has been suggested that the long per- 

 sistence of the genus Dictyonema is not so much a sign of its extreme 



'An extension of this synoptic view on the forms occurring in Canada, which would 

 complete the list for the whole of North America, would according to our present knowl- 

 edge, hardly necessitate any changes beyond the addition of a few Dendroid genera from 

 the Niagaran of Hamilton, Ontario, 



