120 Pertnian Formation of Texas. 
termined by Professor Cope, and in the known superposition of the 
formation containing these faunas upon characteristic Coal-measure 
strata. The first point pf interest relates to the interdelimitation 
of the Mesozoic and Paleozoic ; and the second, to the assumed 
Permian age of the Texan formation from which the collections re- 
ferred to were made. 
The biological interdelimitation of the Mesozoic and Palasozoic 
ages in geological history has long been regarded as clearly recog- 
nizable in all parts of the world. While it was well known that a 
considerable number of generic forms, especially of the inverte- 
brates, respectively occur in strata of both ages, palaeontologists have 
generally regarded it as a fundamental fact that certain orders, 
families, and even genera, which possess certain characteristics of 
structure and form, were rigidly confined to each age respectively. 
That is, they believed that the types which fall into the one cate- 
gory all ceased to exist at the close of the Palaeozoic age, and that 
no member of the other category began its existence before the 
opening of the Mesozoic age. The presence of remains belonging 
to either the one or the other of these categories was therefore re- 
garded as affording unquestionable proof of the geological age of 
the strata containing them. Attempts were made to explain the 
first discoveries of the commingling of earlier and later types in 
one and the same stratum, by assuming that the specimens showing 
the earlier types of structure were derived in an already fossil 
condition from pre-existing strata in the process of their destruction 
by which the materials for new strata were produced. 
However unphilosophical those views concerning the chronolog- 
ical restriction of certain types may appear in the light of modern 
biology, it is not to be denied that until within comparatively few 
years paleontological observations in the field seemed, as a rule, to 
favor them. These later discoveries, important instances of which 
have been referred to, show conclusively that animals belonging to 
both the categories which have just been indicated lived contempo- 
raneously. It furthermore appears that some of those which have 
been regarded as exclusively mesozoic in character began their ex- 
istence while yet Palaeozoic forms were far in the ascendant ; and 
also that many Palaeozoic types survived their earlier associates and 
lived in association with Mesozoic faunas. As I shall discuss this 
subject in another publication, it need not receive further consider- 
ation here ; but I offer in following paragraphs some general re- 
