96 CARBONIFEROUS LIMESTONES OF IOWA. 
Superimposed upon this oblitic bed are more than a hundred feet of crinoidal 
limestones, containing a great variety of beautiful species, belonging chiefly to the 
genera Pentremites, Platycrinus, Cyathocrinus, Actinocrinus, and Poteriocrinus, many 
of which are figured on Table V., A and B. 
For variety and elegance of form, the species afforded by the Burlington beds of 
this subdivision, are probably unrivalled. In the Hannibal beds, they are neither 
so perfect nor so numerous. Interposed between the crinoidal beds of these two 
localities, are white, semicrystalline, and semiodlitic beds, in which the genera 
Productus and Spirifer ave, also, abundant. 
Above these encrinital beds occur the shell-beds, so designated, because especially 
rich in remains of Brachiopoda, particularly of the genera Productus and Spirifer. 
These beds afford, however, a considerable variety of other fossils, Capulus, 
Cardium, Phillipsia, and the teeth of certain fishes, belonging to the genus 
Psammodus, the remains of which contrast beautifully, in their rich, brown 
colour, with the cold, light-gray of the embedding limestone. 
The thin-bedded limestones, which, in the ascending order, succeed these, contain 
a considerable variety, both of Brachiopods and Enerinites ; but their distinguishing 
characteristic is the presence of those screw-shaped, reticulated corals, known as 
Archimedes (Table IV., Fig. 1). This fossil has, in these beds, a vertical range of 
about fifty feet, reaching to the top of the lower series. 
The geodiferous group, at the base of the upper series, contains few, if any, 
organic remains. In the magnesian beds above are found lamelliferous, reticulated 
corals, referable to the genera Fenesiella, Gorgonia, Retepora, and Cellepora. Also 
a Cheetetis, Terebratula lamellosa, and a few other fossils. 
In the gritstones comprising the third group, no fossils have been detected. 
The fourth group, or Lower Concretionary Limestone, is chiefly characterized by © 
a species of Lithostrotion, described by Lesueur under the name of Stylina,* from 
the styloid process which forms the axis of its terminating cells, and usually 
projects conspicuously above their general surface ; especially when the fossil is of 
a cherty nature, as it usually is when occurring in this geological position. 
This fossil coral, it would seem, is as abundant in Iowa as is the Lithodendron 
(Diphyphyllum of Lons.) in the vicinity of Kettlewell; and occupies the same 
geological position. It is also worthy of remark, that though these two corals 
have been referred to distinct genera, both have a prominent central axis, and 
otherwise so closely approximate to each other in external structure, that they are 
not easily distinguishable apart. 
The white and brown gritstones of the fifth group occasionally contain impres- 
sions of Calamites, Lepidodendrons, and other plants of the coal-measures, 
The sixth and uppermost group of the carboniferous limestone is, for the most 
part, destitute of fossils. Locally, a few small Terebratule and Spirifers have been 
noticed. 
* Table IV., Figs. 5 and 6. 
