94 
ANNALS NEW YORK ACADEMY OF SCIENCES 
These Mississippic and Carbonic genera are two of the most persistent 
terminal members of the Streptelasma series, but with the Carbonic these 
forms, too, disappear. This Streptelasma series passes through the stages 
characteristic of all evolutionary series. First starting from very simple 
primitive forms it passes through a long slow period of development during 
which new characters are added little by little. At last the acme of develop¬ 
ment is reached. The stem branches off into divergent and highly specialized 
lines. As soon as this high specialization and divergent development begins, 
the group as a whole seems to lose vitality, and it rapidly declines and 
disappears. A few of the terminal members seem either to have more vital¬ 
ity or to be better adapted to the surrounding conditions than the others 
and they last a little longer, but even these at length are unable longer to 
resist and finally disappear. 
The diagram on the following page shows the probable relationship of 
the various genera and species discussed in this paper. 
