272 THE PALEONTOLOGY OF MINNESOTA. 
{(Dekayella prwnuntia. 
with the two varieties immediately preceding, but specimens are distinguished at 
once by the much greater size of the acanthopores, these being so large and prom- 
inent that they are easily seen by the unassisted eye. An average distance of 0.5 mm. 
separates them. Internally the walls vary in thickness, very commonly even in the 
same tangential section. Where they are thinnest the acanthopores are the most 
distinct, and it is chiefly in such parts that the small set is determinable. Aside 
from the latter the general appearance of tangential sections is decidedly like that 
of typical Dekayia. 
Mus. Reg. Nos. 6016, 7657, 8022, 8098, 
Var. MULTIPORA, 7. var. 
PLATE XXIII, FIGS. 44-47. 
In this form the acanthopores are likewise very large and, as a rule, are to be 
seen with the naked eye on the surface of all well preserved specimens. Still there is, 
as may be seen by comparing figs. 44 and 45, considerable variation in their size, so 
that the identification of the variety depends chiefly upon the unusually numerous 
and large mesopores and the rounded shape of the zocecia. In many cases, however, 
it is difficult to discriminate positively between the two sets of tubes, as these are 
shown in tangential sections. As a rule—perhaps the test is reliable at all times— 
the zocecia never have any part of their walls convex upon the inner side. In 
vertical sections the mesopores are distinguished by having the diaphragms more 
crowded than they are in the zocecial tubes. A radial arrangement of the cells 
about the acanthopores, as shown in fig. 45, is frequently noticeable. Both sets of 
acanthopores about equally numerous, but the smaller set is liable to be overlooked 
except when the walls are unusually thin. 
Mus. Reg." Nos. 6021, 8306. 
These species and varieties gave me more trouble than the whole genus Homo- 
trypa. It seemed impossible to draw up a thoroughly satisfactory classification of the 
hundreds?of Minnesota specimens of Dekayella studied. The separations made were 
generally recognizable, and some of them are based upon not only obvious, but upon 
what, as a rule, we may regard as important structural deviations. Extended inves- 
tigation, however, seemed to show that in the present cases the peculiarities were 
too inconstant to deserve specific recognition. The var. simplex appears to be the 
best marked and most constant, and should, perhaps, have been called a good species, 
with var. nevigera under it. In that case the var. echinata also should be raised to 
