BRYOZOA. 



..- - ; r " 



upward till they graduate into the elongate, confluent zooecial apertures. Arrange- 

 ment and appearance of zooecial 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 

 i< 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 zo-.ecial apertures in 5 mm.: diagonally nine or 

 ten in 2.5 mm.: transversely six of the central rows in 1 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 zooecial 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 figa. 23 and 24. These 

 figures show further that the apertures are now wider, with only fire in 1 mm. 

 transversely, and of snbrhomboidal 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 zoarinm. 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 zooecial apertures foreshadowing true 

 Ptil&iittya. 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 

 zoarinm. 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 

 zoo?cial aperture-. 



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.) 



