"^0 North American 



Pkoetus. 



1. Glabella tumid, gibbous in front, but not overhanging its anteiior 



border. 



2. Marked by three pair of short lateral furrows. 



8. Basal lobes separated from the glabella by very distinct furrows. 

 4. The axis of the pygidium is always raised above the margui and di- 

 minishes to a blunt extremity ; it has from 4 to 18 segments. 



Phillipsia. 



1. Sides of glabella nearly parallel. 



2. Marked by either two or three short lateral furrows. 



3. Basal lobes continuous with the glabella. 



4. Eyes large reniform. 



5. Pygidium usually with a ])order. 



6. Axis composed of from 12 to 18 segments. 



GRIFFITHIDE!; 



1. Glabella pyriform. 



o 



No short lateral furrows on the glabella. 



0. Basal lobes distinct from the glabella. 



4. Eyes small, suboval. 



5. Pygidium rounded, composed of from 10 to 18 segments. 



Brachymetopus. 



1. Glabella short, tumid. 



2. No short lateral furrows on the glabella. 

 8. Basal lobes distinct. 



4. Eyes small, placed close to the glabella, reniform. 



5. The axis of the pygidium has from 10 to 17 segments, 



II. — Classification of the Oarbonifekous strata of the 

 Ujs-ited States. 



Lower Carboniferous. — In the United States, this series is 

 represented in the West along the valley of the Mississippi River 

 by a limestone deposit, but in the Eastern States these limestones 

 are replaced by a sandstone cind conglomerate, Avith occasional 

 beds of limestone, like that of Maxville, Ohio.' The Lower Car- 

 boniferous sand beds of Pennsylvania thin away and gradually 

 disappear before they reach the Mississippi, while the Chester and 



1 Geol. Sur. Ohio, Vol. 3, p. 821. 



