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1895. | NEW YORK ACADEMY OF SCIENCES. 263 
It is not many years since we were accustomed to look upon 
the Quebec Group as containing the first true graptolites, mean- 
ing thereby the Rhabdopora, but the discoveries of the last ten 
or fifteen years make it necessary to modify this conclusion. 
Not to mention the species represented by obscure or imper- 
fectly preserved remains of the Lower Cambrian (Olenellus and 
Paradoxides Zones), there is obviously at a higher horizon in 
the Cambrian System a fair representation of the Rhabdopora, 
though not the wealth of forms which meet us at the threshold 
of the Ordovician System. 
The graptolites of the Upper Cambrian centre around the form 
known as Dictyonema flabelliforme Hichwald (=D. socialis 
Salter), which in its earlier stages was actually a twig-graptolite 
and only in its later stages acquired the reticulate habit. This 
widely spread and well known species is of value as marking a 
definite stage, 7m the Cambrian as understood by English Geolo- 
gists, but at the summit of this System as limited by those of 
the continent (Europe). 
Still though the horizon of Dictyonema flabelliforme is well 
known, the extent to which these Cambrian Rhabdopores are as- 
_ sociated with it does not seem to have been clearly ascertained 
in Europe, for by some authors they are placed a little earlier in 
time, and by others a little later than the above species. As 
regards their position in America, however, there is no question, 
as they occur in intimate association with Dictyonema flabelli- 
forme,and that at more than one level. This inclusion in the 
Dictyonema Zone, apparently different from the conditions of 
occurrence in Europe, may be due to the fact that the species 
above named has a considerable vertical range on this side of 
the Atlantic. In Europe, and especially in Scandinavia, this Dic- 
tyonema has its place above the trilobitic fauna associated with 
Peltura scarabeoides, technically known as the Upper Olenus 
Zone, but in the Canadian Cambrian beds we find this Upper 
Olenus Fauna extending into the Dictyonema Zone, since the 
trilobites of this fauna occur in lentiles with layers included in 
shales which contain this Dictyonema. Our practice, however, 
has been to regard the beds above the horizon to which these 
trilobites so far as known are limited, to be the true zone of 
Dictyonema, and corresponding to the beds so designated in 
Europe; and to include the beds below, in which Leptoplastus, 
Spherophthalmus and Peltura are found, and which also con- 
tain Dictyonema, as a lower zone corresponding to the upper 
part of the Upper Olenus Zone as developed in Wales and 
Scandinavia. Further remarks upon this subject will be found 
in a later part of this paper, where the several species of Rhab- 
dopora are described. 
