224 Observations upon the Geology and Paleontology 



of those mucronate hinge extremities so common to the 

 Devonian Spirifers. The S. Forbesi approaches the latter 

 form most nearly, but it reaches its greatest development 

 but a few feet above the range of Devonian fossils, and 

 perhaps should be considered as an intermediate species. 



By this separation of the fossils of the Burlington rocks, 

 the Brachiopods of the lower beds are found to bear less 

 general resemblance to those of the Burlington limestone, 

 immediately overlying them, than they do to those of 

 almost any other member of the Carboniferous limestone 

 series, notwithstanding the fact that the Chemung and 

 Burlington limestone beds are otherwise so intimately 

 related. Indeed, quite a number of species could be 

 selected from the Brachiopods of the Chemung beds at 

 Burlington, that bear so close a resemblance to others of 

 the upper members of the Carboniferous limestone as to 

 require considerable acquaintance with them to distin- 

 guish their differences. 



The contrasts and similarities above mentioned seem to 

 have taken place without any particular regularity or gra- 

 dation, but by comparing the Crinoida of the lower and 

 upper beds of Burlington limestone, together with those 

 of the Keokuk limestone, we find that they present three 

 successive grades of development, which, commencing in 

 conjunction with Devonian forms, furnish otlier links by 

 which the Devonian rocks are connected with the Carbo- 

 niferous. The crinoids of the lower bed of Burlinifton 

 limestone are generally of smaller size than those of the 

 upper bed, and have a delicacy of construction and sur- 

 face-marking and ornament, together with a pleasing 

 symmetry of outline, which present quite a contrast to 

 those of the upper bed, when considered in the aggregate. 



Those of the upper bed are generally of stronger con- 

 struction and ruder form, and have a coarser style of 

 ornament. 



