PHYSICAL AND FAUNAL EVOLUTION 57 



Didymograptus is represented by three species/ all common in the 

 Normanskill, and all distinct from those of the lower horizons, where 

 eighteen species are recorded. Of the genera beginning in the third 

 Deepkill, or Point Levis zone, Climacograptus has only one species 

 in the lower zone, which is not known above that zone, while there are 

 thirteen species, most of them abundant, in the Normanskill; Crypto- 

 graptus has one species in the lower and two others in the higher 

 zone, common in each case; Desmograptus has two species in the 

 lower and one in the higher, the latter rare; Diplograptus has four 

 species in the lower and thirteen in the higher horizon, all distinct; 

 while Clonograptus has two rare species in the lower and nine in the 

 upper, mostly common. It is thus seen that there are no species 

 in common between the two zones, and the most characteristic 

 genera of each are unknown or rare in the other. On the other hand, 

 six out of the twenty-four species listed by Ruedemann for the third 

 Deepkill zone, or 25 per cent., occur also in one or both of the lower 

 zones. Its relationship to that and distinctness from the Norman- 

 skill zone thus becomes evident. The forty feet of the third Deepkill 

 zone probably represents the last deposits in an already shoaling and 

 contracting channel before interruption took place, this break con- 

 tinuing to the end of Chazy time, when a new graptolite fauna came 

 into existence.^ 



On the whole, the Beekmantown represents one of the large 

 stratigraphic divisions of the Ordovicic of North America. Its 

 fauna is essentially a unit, and although the succeeding Chazy fauna 

 is in part, at least, derived from the Beekmantown, its distinctness is 

 nevertheless marked. The Beekmantown corresponds to a great 

 negative diastrophic movement, with the exception of the lower por- 

 tion, and its thickness (2,500 feet where fully developed) shows that 

 it represents fully one- third of the entire Ordovicic series, and pre- 

 sumably represents one-third of Ordovicic time. From this it 

 follows that the Beekmantown alone represents the Lower Ordovicic 

 in North America, the Middle Ordovicic beginning with Chazy 

 deposition. The term Beekmantownian has therefore been proposed 

 as the North American equivalent of Lower Ordovicic, while the 



^ Varieties are here classed as species. 



2 See Ruedemann, Graptolites of New York, Vols. I and II. 



