SPIRIFER^ OF THE HAMILTON GROUP. 233 



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- 

 tetnes 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 xxxviii and xxxviii 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 8. medialis 

 are at least ten times as numerous as those which can be referred to either 

 of the other species. 



[ Pxl-iEONTOLOOr IV.] 30 



