126 Permian Formation of Texas. 
and their geographical range includes both the Illinois and Texan 
vertebrate localities. That is, the invertebrate fauna referred to 
is uniform over a region in which the vertebrate fauna is diverse. 
In all the vertical and geographical range of these invertebrate 
fossils, there has never been observed any evidence of the deca- 
dence of old forms^^ such as would be taken to indicate an approach- 
ing close to the geological period which the}- have especially char- 
acterized ; and it is only in the case of the Texan Permian that an 
introduction of new forms has been yet observed which might be 
regarded as forerunners of a new one. 
Finally, while it is freely admitted that a considerable number 
of the invertebrate species which characterize the Permian of Eu- 
rope have nearly related representatives on this continent, it should 
not be forgotten that they are as characteristic of our undisputed 
Coal-measures as of the reputed Permian. Even if those forms are 
really specifically identical on the two continents it does not neces- 
sarily prove the contemporaneity of the respective formations con- 
taining them. In fact those formations must be necessarily of a 
difference in age equal to the time required by the distribution of 
The recognition of the Permian of Texas as a separate upper 
group of strata belonging to the Carboniferous system is based up- 
on both stratigraphical and palaeontological evidence, and this evi- 
dence is fuller than that which has been adduced in favor of any 
other reputed Permian strata of North America. First, it contains 
invertebrate species which have been referred to the Permian in 
other districts to the northward, some of which are closely related 
to Permian species of Europe. Second, it contains the 
large vertebrate fauna published by Professor Cope, which he 
regards as characteristically Permian. Third, the Texan formation 
evidently constitutes an upper, apparently the uppermost, portion 
of the Carboniferous system. Fourth, the lithologica) difference 
between this formation as a whole and the Coal-measures beneath it 
other spe:ies which characterize the Coal-measures, have never been found in any 
of rhe reputed Permian strata, and it seems to have been assumed that their ab- 
sence was due to a final decadence of those forms before the Permian period was 
reached. It seems, however, not at all unreasonable to infer that successive 
iifferently affected different classes of animals, in conse- 
ms referred to were not extinguished, but only differently 
