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ANNALS NEW YORK ACADEMY OF SCIENCES 
fundum and then acquires the characters which are distinctive for this 
species. It is further to be noted that the fully developed septa never unite 
at the center to form a pseudocolumella. Only the incompletely developed 
septa are attached by their inner margins to the next preceding septa. 
Streptelasma rusticum passes through the same development as S. corni- 
culum but is distinguished from it in the adult stage by its long cylindrical 
manner of growth; the latter is cone-shaped throughout its life while the 
former is cone-shaped during its early life but becomes cylindrical in its 
adult condition. 
It is appropriate at this point of the discussion to make note of a paper by 
F. W. Sardeson published a few years ago, entitled “On Streptelasma profun- 
dum (Owen) and S. corniculum, Hall. 1 ” In this paper, Sardeson advocates 
including under one species, Streptelasma profundum, twelve or more species 
of this genus and three or more species of the genus Zaphrentis described 
from the various Ordovicic limestones of the United States and Canada. He 
notes the great difference in size of the geologically early and later forms 
and the difference in rate of expansion or angle of the apex in the different 
species and also that certain forms such as S. corniculum are always conical 
or horn-shaped, while others like S. rusticum become cylindrical, yet he 
believes that these should all be considered simply as individual differences 
due to the length of the life of the particular individual. He contends that 
they should all be grouped as one species, because individuals indicating 
gradations from one condition to another can be found. In arguing from 
this that they should all be one species he ignores the fundamental concep¬ 
tions of all animal evolution, namely: that if all the individuals of any genetic 
series were preserved, there would be no sharp line of distinction or demarca¬ 
tion between one species and another but that an unbroken series of inter¬ 
mediate forms would be found which would show all stages of the change 
from the one species to the other. To the mind of the writer the amount of 
change in passing from Streptelasma profundum to S. corniculum and from 
this to S. rusticum is sufficient to constitute a valid specific distinction. The 
numerous other species of Streptelasma from the Ordovicic have not been 
studied, but probably most of them are valid species also. 
The Streptelasma profundum-corniculum-rusticum line of development 
in so far as the Streptelasma stem is concerned seems to close with the Ordo¬ 
vicic. Perhaps this line of development in the Devonic gives rise to the 
Cyathophyllum stem. The present investigation has not been carried far 
enough either to prove or to disprove this supposition. 
During the Siluric period the Streptelasma line of development advances 
1 American Geologist, Vol. XX, Nov., 1897. 
