Permo-Carboniferous Ammonoids of the Glass Mountains 207 



carboniferous of Central Texas and their relations to other strata of 

 the earth. 



In the Permo-carboniferous of Central Texas we know at the 

 present time at least five different horizons characterized by am- 

 monoids. All of these have in common the preponderance of Perrin- 

 ites belonging to different species. 



The succession of these horizons is as follows : 



5. Horizon of Quanah with large Perrinites and large Gastrioceras. 



4. Horizon of Salt Croton Creek with Perrinites Hilli and undeseribed Popan- 



oceras, Medlicottia, etc. 

 3. Horizon of Runnels County, with Perrinites n. sp., Medlicottia n. sp. I, 



M. n. sp. II, Gastrioceras n. sp. 

 2. 'Horizon of the old Military Crossing of the Wichita, with Perrinites Cwnv- 



minsi, Medlicottia Copei, StacJieoceras Walcotti, Paralegoceras Baylorense 

 1. Horizon of Dundee with Perrinites n. sp., Medlicottia n. sp, Agathiceras 



sp. ind., StacJieoceras n. sp. 



While the youngest horizon has been found rather high up in the 

 Double Mountain formation, the lowest one seems to be still far above 

 the limit between the Carboniferous and the Permo-carboniferous. 

 Possibly there exists another horizon near San Angelo, Tom Green 

 County, but only a Medlicottia has been cited from this locality, which 

 is probably below the Runnels County locality. 



We have seen that those genera which are found in several of 

 these five horizons show a distinct development of species from simpler 

 to higher stages of the suture ; these genera are principally Perrinites 

 and Medlicottia and in second line, Stacheoceras. If we now extend 

 our comparisons to the Trans-Pecos region of West Texas, we find 

 that our lowest horizons i and 2 have not yet been identified there, 

 while on the other hand, the lowest horizon with Uddenites of the 

 Glass Mountains has not yet been found in Central Texas. But our 

 horizon 3 may possibly correspond with the lower part of the Leonard 

 formation (horizon of Perrinites) ; at least the difference in age can- 

 not be very great. Our upper horions 4 and 5 certainly correspond 

 exactly to the horizon of Perrinites vidriensis or the upper Leonard 

 formation. The Word formation is certainly younger than any of the 

 ammonoid-bearing horizons so far found in Central Texas. The 

 species of Medlicottia described from the Leonard and Word forma- 

 tions are clearly much younger and higher developed forms than those 

 found in our horizons i, 2, and 3. They are forms with distinctly trilo- 



