206 The American Geologist. April, 1902. 
founded. It may be well therefore to discuss the subject at 
this point, since the evidence presented by the form just de- 
scribed is so unequivocal. 
In his observations on Dekayella, quoted above, Mr. Ul- 
rich says that the large and small sets of acanthopores must 
differ considerably in function. I cannot believe that this is 
the case. As is shown in this paper the two sets are present 
in practically all members of the genus Dekayia (includes 
Heterotrypa and Dekayella); and moreover there is every 
gradation even in Dekayella ulrichi between conspicuous differ- 
ence in the size and relative abundance of the two sets of acan- 
thopores in some specimens, and very little difference in oth- 
ers. The maximum difference, so far as I have noticed, oc- 
curs in Dekayia aspera, where the smallest acanthopores are so 
minute (fig. 10, pl. X) that they must usually escape notice, 
except in sections very carefully prepared and ground as thin 
as is compatible with retaining structural details. In some 
unequivocal specimens of Heterotrypa inflecta from Vevay, 
Indiana, the appearance of the acanthopores, which are very 
abundant, is just the same as in Dekayella ulrichi or D. ulrichi- 
robusta, i. e., a few very large ones and numerous very small 
ones. In fact had it not been for the thin flat frond, inflected 
apertures of the zocecia, and the occurrence of the specimen 
in the Platystrophia zone, I should at the time most certainly 
have referred it to one of these species ; and I am not sure even 
now but that it would be better to consider Heterotrypa inflecta 
as a variety of Dekayella ulrichi (very close to D. ulrichi-lo- 
bata and D, ulrichi-expansa. ) 
[ believe that the difference in size of the acanthopores as 
seen in tangential sections is due solely to the fact that some 
are cut nearer their point of origin than others. The large 
ones having originated earlier in the life of the colony, have 
attained to large size because they have attained to maturity. 
The small ones originate in the peripheral region and though 
they may enlarge rapidly, the section is quite likely to cut them 
near their point of origin where their diameter is necessarily 
small. If the Heterotrypidae are derived from an ancestor 
in which acanthopores were lacking, these structures must first 
have made their appearance in small numbers in the peripheral 
region and then have appeared earlier and earlier in succeed- 
