Permian Formation of Texas. 119 
collections, no American palorentologist who is familiar with the 
Coal-measure fauna, would probably have hesitated to refer them all 
to that period. 
It is doubtless true that because so large a proportion of the in- 
vertebrate species, which have been obtained from reputed Permian 
strata in North America, occur also in characteristic Coal-measure 
strata, no satisfactory sepa-ation of them into two groups has hither- 
to been practicable upon the evidence of invertebrate fossils ; and 
stratigraphical evidence has hitherto been unsatisfactory also. The 
collections, however, which are represented by the foregoing list and 
descriptions, although consisting mainly of Carboniferous forms, con- 
tain at least two types which are so generally regarded as indicating 
the Mesozoic age of the strata containing them, that if they alone, 
and without any statement of correlated facts, had been submitted 
to any paleontologist, he would not have been warranted in refer- 
ring them to an earlier period than the Trias, if he had followed the 
usually accepted standard of reference. These two forms have 
been described on preceding pages, under the names of Ptychites 
cumminsi and Popanoceras 7valcotti respectively ; and with the ex- 
ception of the Ammonites Parkeri" of Heilprin, also from Te.xas, 
similar types have never been found associated with recognized 
Carboniferous species in North America. 
This, however, is by no means the first, nor the most important 
discovery of the commingling of Mesozoic and Paleozoic types in 
such a manner as to indicate that they all lived contemporaneously, 
and were members of one and the same fauna. The remarkable 
discovery by Professor Waagen, in India, oP many molluscan spe- 
cies belonging to mesozoic types associated with a characteristic 
Carboniferous fauna is well known. It is also well known that mes- 
ozoic characters are recognizable among certain of the Carbonifer- 
ous and Permian cephalopods of Russia and Armenia, as well as of 
certain parts of Europe. 
The special interest which these Texan collections possess lies, 
first, in the presence of the two cephalopods of mesozoic type as 
members of an invertebrate fauna composed otherwise of paleozoic 
types ; and second, in the association of this invertebrate fauna 
with a vertebrate fauna composed mainly of Permian types, as de- 
* Proc. Acad. Nat. Sci. Phila. 1S84. vol. XXXVI. p. 53- 
' See Paleontologia Indica Series XIII ; Salt Range Fossils. 
