Paleontological Speculations. — Gratacap. 
223 
but slightly. In the evolutionary change from one organic 
form to some possible organic deduction from it, the gener- 
ative organs are probably the last to become fully habituated 
to an altered form and nature. There would certainly be less 
violent change from the generative functions of a worm to 
those of a trilobite than from the same to those of a brachio- 
pod, and along some line of elucidation like this we may par- 
tially explain the early preponderant profusion of trilobitic 
forms. 
Another instance of a biological crisis is the abundance of 
lamellibranch remains in the Upper Devonian in the Hamil- 
ton and Chemung rocks of New York. There is presented in 
the Xew York Hamilton, an extraordinarv development of 
lamellibranchs exhibiting a marked and almost sudden dis- 
play of genera which continues with an increased numerical 
intensity and a noticeable similarity of types into the Chemung, 
to be practically obliterated in the Lower Carboniferous. The 
phenomenon seems also to distinguish an optimum of condi- 
tions for growth and variation, and, as in the case of the tril- 
obites, must have resulted from a conjunction of a favorable 
environment and vital force. There is certainly a well marked 
development of bivalves in the Devonian and along with the 
appearance of new genera, a multiplication of species and an 
immense increase in individuals. 
In the studies of paleozoic lamellibranchs so carefully pre- 
pared of the N. Y. Geological Survey, though unfortunately 
incomplete, a tabulation of the species from the different geo- 
logical horizons from the Schoharie to the W'averly aft'ords 
this interesting summary : 
V 
u 
bfl 
!/l 
rt 
ai 
u 
<u 


CLi 
u 
o 
16 
41 
21 
174 
252 
35 
The showing of species for the Waverly is misleading. In 
the Preface (Vol. V, pt. i.) the authors (Drs. Hall and 
Beecher) explain the comparatively few species from this hori- 
zon as due to delay, interference, and their solicitude to bring 
their work to completion and publication. 
