Permo-Carhoniferous Ammonoids of the Glass Mountains 201 



and that another part of it persisted in the more archaic form through 

 several horizons. 



We shall presently see that such development can also be shown to 

 exist in the species of Perrinites in Central Texas. 



Medlicottia Copei has been cited from higher horizons in Central 

 Texas, but as long as the species is not described in detail nor figured, 

 we cannot draw a conclusion, as these determinations may only be 

 provisional and a close study of those forms may show that in reality 

 they belong to different species. 



The two new species from Trans-Pecos country, described in this 

 paper (M, Whitneyi and M . Burckhardti) belong certainly to a different 

 group. They are much more similar to some Sicilian species from the 

 Sosio limestone and show an entirely different form of the saddles, 

 which are distinctly trilobate. 



We see that at least in Texas it seems to be possible to distinguish 

 the different horizons in the Permo-carboniferous by the different 

 form of development of Medlicottia and that possibly Haug and Noet- 

 ling may not have been so far off of the truth, when they contended 

 that such was the case generally ; although this idea has been declared 

 to be wrong by several distinguished authors. If we take into account 

 the development of Medlicottia in higher stages of the Permian and 

 the Triassic, including also Episageceras, with the extension of our 

 knowledge of those forms, we may find that there is a very definite 

 order of development in this tribe of the ammonoids. When Haug^ 

 first conceived the idea of subdividing the Permian into zones based 

 on the different groups of Medlicottia, our knowledge of these forms 

 was still very restricted and it therefore was too early to undertake 

 this task; but at the end it may be shown that Plaug's idea was en- 

 tirely right, and that he only did not have sufficient material, at that 

 time. 



Perrinites n. sp. 



Unfortunately, the numerous casts and molds collected by Beede 

 are so fragmentary and badly preserved that a complete description 

 of. the species remains impossible, for the moment; even the suture 

 can be observed only partly and on different specimens. But on the 



'Haug, Et. s. 1. Goniatites, p. 70. 



