222 AMADEUS W. GRABAU 
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 Beekmaniown 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 
t Varieties are here classed as species. 
2 See Ruedemann, Graptolites of New York, Vols. I and II. 
