82 Calvin on the Hamilton in Ontario r 
fossils on which the Iri-partite division of the shales is based^ 
nor will any particular zoological order be followed. All that 
can be done will be to refer to the more obvious and more 
characteristic forms of each separate group. 
We find the lower division of one section exposed, just above 
the level of the water in the river, below Bartlett's Mills. It 
may be traced down the river for a number of miles. Near 
Arkona a little stream flowing toward the Aux Sables has cut 
a gorge in the shales, known as Rock Glen ; and a short distance 
above the mouth of the stream the lower division is again finely 
exposed. 
One of the most characteristic forms of the lower division is. 
the long-winged variety of Spirifera mucronata Conrad.' It is 
true that Spirifera inucronata is the most abundant and most 
characteristic species of the third or upper division, but the 
varieties from the two horizons show much greater differences 
than are often observed between two well-established species. 
This long-winged variety — the variety d. of Nicholson, (Pal, 
of Ontario, 1874,) — has the hinge line greatly extended so that 
the width is not unfrequently five or even six times the length; 
the valves are flat, giving the shell a compressed appearance; 
the plications on each side of the mesial fold and sinus are small 
and numerous; the mesial fold is divided by a groove and the 
sinus by a median angular ridge. 
Along with the characteristic long-winged Spirifer are asso- 
' The name, Sfirifera mucronata^ is here used with full knowledge of 
the fact that there is a name twenty-one years older than it, applied to 
this same species. In 1878 S. A. Miller (Proc. Dav. Acad. Nat. Sci., vol. 
ii, p. 220,) called attention to a neglected article by Mr. Caleb Atwater, in 
Am. Journal of Science for 1820 (vol. ii, p. 244). In that article Mr. 
Atwater describes and figures a fossil species under the name of Tere- 
bratula pennata; and, as Mr. Miller says, a reference to the description 
and figures leaves no doubt that the author had before him the species 
that Conrad, twenty-one years later, named Delthyris mncronata. The 
species, therefore, in strict justice should be Sfirifera fcniiafa, Atwater, 
but I find it difficult to overcome a natural aversion to disturbing a name 
as well established in usage and literature as that of Conrad. 
The revival of Atwater's name for the species, so long associated with 
the name of Conrad, would compel a change in the Spirifcra pennata of 
Owen, and for this latter species Miller proposes the name Spirifera 
atwaterana. 
