SPIRFFERiE OF THE HAMILTON GROUP. 
Add to this resemblance in general form, proportions of area, etc., the 
great-similarity of surface characters, and the question seems scarcely to 
admit of a solution without uniting all these as one species. 
Regarding S. angusta as the young condition, we need only a less 
development and arcuation of the area, with coarser plications, to make 
it undistinguishable from a young S. medialis. Carrying on the develop¬ 
ment in the same direction, shortening the cardinal extremities and 
increasing the gibbosity of the valves, we have the S. medialis in its 
typical form. Going still farther in the same direction, the cardinal 
extremities may become rounded, the valves ventricose, and the area 
arcuate, with still stronger plications, giving the var. eatoni. 
In the younger forms we have those with the area slightly arcuate, 
vertical and receding. If we regard these features as only conditions of 
the same species, we may have those with the vertical areas developed 
in the same direction, while the cardinal extremities continue much 
extended, until we have the typical form of S. macronota. These forms 
continuing, the area vertical or slightly inclined but scarcely arcuate, 
may have the cardinal extremities shortened and the shell becoming 
ventricose, presenting form and characters which it is equally difficult to 
refer to either S. medialis or S. macronota. Again, we find here and there, 
among the collections of Hamilton Spirifers, a form where the area is 
vertical as in S. macronota, but the lateral extensions much less, and the 
ribs fewer and perhaps a little stronger, suggesting a relation to S. euru- 
teines and S. manni of the Corniferous limestone. 
I am not at present prepared to assert the identity of all these forms; 
but I can easily believe that larger and more extended collections, made 
over geographical areas not yet explored, may show a much closer rela¬ 
tion than we have supposed to exist between them. 
I have endeavored, in Plates xxxvm and xxxvm a, to show all the 
important varieties of form, selected from a collection of several hundred 
individuals, and in which the specimens readily referable to S. medialis 
are at least ten times as numerous as those which can be referred to either 
of the other species. 
[ Paleontology IV.] 
30 
