BRYOZOA. 169 
Escharopora. | 
upward till they graduate into the elongate, confluent zocecial apertures. Arrange- 
ment and appearance of zocecial apertures and interspaces varying with age. In young 
examples, or those less than 25 mm. in length, and these seem to be by far the most 
abundant, the surface appears as in figures 18 to 21. In these the zocecial apertures 
over the entire surface, excepting near the base and along the edges, are elongate 
elliptical, sometimes almost acute at the ends, and arranged between alternately 
converging and receding ridges, which, failing to close around the ends of the 
apertures, permit confluence between them through a narrow channel. The result 
is a very pronounced longitudinal arrangement, though the diagonal rows, and 
sometimes the transverse as well, are scarcely less evident and regular. Measuring 
lengthwise there are about eleven zocecial apertures in 5 mm.; diagonally nine or 
ten in 2.5 mm.; transversely six of the central rows inl mm. The marginal rows 
are always larger, and occasionally have the oblique character shown in fig. 21. In 
specimens 25 mm. long the upper extremity will already indicate the changes that 
took place in later growth. The shallow channel connecting the zocecial apertures 
is gradually lost through the closer convergence of the enclosing ridges, till at last 
we have a simple ridge-like separating wall as shown in figs. 23 and 24. These 
figures show further that the apertures are now wider, with only five in 1 mm. 
transversely, and of subrhomboidal or hexagonal form. The increased width is 
accounted for partly by the loss of the channels, and the remainder by the increased 
circumference of the zoarium. The largest specimens usually exhibit a central row 
of small monticules. In some there are: two irregularly alternating rows, while in 
the fragmentary original of fig. 17, there are three rows. The last specimen is 
peculiar also in having an arrangement of the zocecial apertures foreshadowing true 
Ptilodictya, namely, oblique “pinnate” rows predominate on the spaces between the 
outer monticules and the edges of the frond, yet over the central part of the surface 
the usual diagonally intersecting series prevail. 
All the changes produced by age are chiefly apparent in the upper half of the 
zoarium, the appearance of the early stages being more or less preserved in the 
basal portions. Still, very old and thick examples, like the original of figs. 15 and 
22, are likely to develope mesopores here in place of mere channels between the 
zoccial apertures. 
Of internal characters shown in the excellent and instructive sections illus- 
trated, I wish to point out (1) the elongate form of the primitive cell, (2) its shape 
just before being roofed over to form the primitive aperture, and (3) the two at first 
distinct then coalescing lines in the transverse interspaces. (See upper parts of 
figs. 25 and26.) 
