164 THE PALEONTOLOGY OF MINNESOTA. 
F [Ptilodictya. 
The divisions are perfectly natural, and each based upon readily detected pecu- 
liarities. In the first, including P. lanceolata Goldf., sp., and therefore Ptilodictya in 
the strict sense about to be proposed, we have a character that is wanting in the 
second: Namely, a variable number of regular longitudinal rows of zocecia running 
through the center of the fronds from the pointed articulating base upward. In the 
earliest species having this peculiarity, these longitudinal central rows do not always 
extend through the frond to its upper extremity, but they are sometimes found to 
pass into the diagonal arrangement prevailing over the lateral portions of the surface. 
In P. magnifica Miller and Dyer, for instance, the longitudinal rows obtain only in the 
middle of the lower half of the full grown zoaria, the diagonal arrangement being 
present on all other parts of the surface. ' 
These central zocecia are oblong-quadrate in shape, narrower than the lateral 
ones, and always the first to be developed. In the youngest examples of all the 
species they alone occur, and it is only in later stages that the differently arranged 
and wider lateral zocecia are developed. It is possible that this condition, which, as 
said, is an immature or youthful one in most of the species, may have persisted in 
some, and that in these no lateral zocecia were produced. P. gladiola Billings, and 
P. flagellum Nicholson, may be said to support this view, only longitudinal Zocecla 
being as yet known of them. Still, as the evidence is merely negative, and in the 
light of facts brought out in a study of complete suites of P. variabilis of the Hudson 
River group, I am obliged to regard the matter as doubtful. - 
Used in this restricted sense Ptilodictya admits of subdivision into two groups, 
both obvious enough, but, as they now appear to me, not quite natural. In the 
first, with P. lanceolata as the type, we have either nothing but longitudinal rows of 
zocecia, or these are flanked on each side by spaces of greater or less width 
over which the apertures are arranged in an oblique manner, giving the fronds the 
fancied resemblance to a feather that suggested the generic name. The lateral rows 
proceed to the edges of the zoarium without interruption from either groups of 
large cells, monticules, or macule. 
In the second subdivision, and of this P. magnifica M. & D. may be considered 
as typical, the zocecial apertures on the lateral extensions of the zoarium are 
arranged in diagonally intersecting series, with clusters of large cells, monticules, 
or macule, at regular intervals. The pinnate arrangement of parts prevailing in 
the lanceolate subdivision is therefore scarcely recognizable in this, but the presence 
of monticules is an even more striking peculiarity. 
In the second division, for which I propose to adopt Hall’s name Hscharopora,* the 
*T have some slight doubts respecting the specific characters of E. recta, Hall, the original ty pe of the genus, but none 
whatever so far aS its generic characters are concerned, 
