Palacontological Speculations.—Gratacap. 297 
In the Spiriferidz the extension of the hinge line and the 
widening of the cardinal area are evident features which admit 
theoretically of great development. They attain it. The 
former can be traced from S. niagarensis to marcyi, ligus, dis- 
junctus, mucronatus, the latter in S. macrothyris, audaculus, 
Syringothyris typa. 
The shells of the Trematospiridae, Nucleospiridae, Zygo- 
spiridae suggest Spirifer in outline, but might be readily mis- 
taken for them, in some species, but they never exhibit such 
limital forms as are found in the Spiriferidae. These formal 
tendencies do not develop in them. If these limits of Spirifer 
have been produced by strain, elision, conformity, etc., why 
have not such predicaments worked similar results in them? 
Their biological plasticity does not allow it. In short there 
seems apparent in fossils an inherent character which admits 
of modification in certain directions, prescribed by their’ na- 
ture; and different groups move on under the impact of en- 
vironment or some organic impetus resident within them, to- 
wards limital forms, participation in which are denied other 
groups not adapted for these formal tendencies. 
A review of a series of fossil forms through many forma- 
tions establishes the conviction that evolution is not circum- 
stantial, but presents a certain inevitableness in the strict and 
graded tendencies of form. Again such tendencies are also 
evinced in the extreme exaggerations shown in limited faunas, 
as the crinoidal life of the Keokuk beds. 
Simultaneous Faunas. 
Dr. Weller has (Journal of Geology) illustrated two types 
of faunas, (1) the cosmopolitan; (2) the provincial faunas. 
He says: ‘During Silurian time, for instance, there seems to 
have been in existence a great cosmopolitan, shallow water, 
marine fauna, which is represented in America, in Europe, in 
Australia, and New Zealand, and probably will be discovered 
also in the other continents. The conditions for the develop- 
ment of such a fauna seem to have been the presence of wide- 
spread, shallow seas upon the continental platforms, epi-conti- 
nental seas, as they have recently been called. With broad 
seas of this sort around the borders of the continents, and ex- 
