Paleontological Speculations. — Gratacap. 227 
Pararca Pararca 
Palacoiieilo Palaeoneilo 
Prothyris ? 
Ptychopteria 
Pteronites 
Phthonia 
Pholadella Pholadella 
Ptychodesma Ftychodesma 
Paloeanatima 
Palaeoniva 
Prorhynchus 
Solem\-a 
Sphenotus Sphenotus 
Schizodus Schizodus Schizodus 
Solen 
Sangninolites 
Tellinopsis 
The genera of the Upper Helderberg are twenty-one, of 
the Hamilton forty-three, and of the Chemung' thirty-five. 
There is a sensible identity in the genera of the last two form- 
ations, with an increase of species in the Chemung. There 
would seem to be indicated here a biological crisis ; that a 
group of organisms enormously expanded under favorable 
circumstances, and that when they were forced out of their 
habitats, they came into competition wath each other in a 
smaller environment, and upon each return, brought back an 
increased number of species, possibly referable to that com- 
petition as a cause. 
As in the case of the trilobites, this crisis suggests some- 
thing like iinpiilsc. -z'is a tcrgo, assisted indeed by natural 
auxiliary conditions. There would seem tO' be no especial 
reason why, from the Potsdam to the Devonian, the lamelli- 
branchiate phylum, which was certainly in evidence through- 
out that long time, might not have reached as wide a circum- 
scription of species and genera as it did in the last named 
era, any more than there seems any special reason why the 
trilobites in the Cambrian attained a development quite un- 
rivalled since. It has been suggested in this paper that ev- 
olution could have acted more rapidly in the production of 
trilobites than it did in the associated life, reinforced by very 
favorable circumstances for their multiplication. Can it be 
conceived that in the case of the lamellibranchs there were in- 
herent obstacles to evolution, and a protracted restraint en- 
sued before this life group could store up enough biological 
spring to jump forward into formal (generic and specific) 
diversity? Is evolutional aptitude, like individual endowment 
