CARADOCIAN CYSTIDEA FROM GIRVAN. 459 



inff nearly every known species, I find nothing to correct in Dr Jaekel's paragraph 

 on the subject (1899, p. 234). 



S 386. The structure of the proximal region is clearly shown in section in a 

 weathered specimen of P. squamosa (Brit. Mus. E 16045; text-figs. 63, 64). It has 

 a wide lumen, enclosed by flanged columnals, alternating with plain columns over 

 which the flanges project. The body of each type of columnal is of about the same 

 height, but the plain columnals may be, if anything, a little higher ; the body of the 

 flanged columnals appears to thicken very slightly towards the lumen, while that of 

 the plain columnals correspondingly thins. In a well-preserved specimen the flanges, 

 which project both upwards and downwards, often overlap the intermediate columnals 

 to such an extent as almost to hide them ; and, if the stem be bent, then the flanges 

 meet on the concave side, so that the plain columnals are visible only on the convexity. 

 The flanges are nearly always granulate, and in most species the granules fuse into 



Fig. 63. Fig. 64. 



Text-figs. 63 and 64. — Portions of the stem of Pleurocystis squamosa, with a quadrant 

 removed to show the section. The diagrams are based on Brit. Mus. E 1604.5, 



Fig. 63.— Represents the proximal region. 



Fig. 64. — Represents the passage from the proximal to ths distal region. 



vertical ridges. Granules may also be borne on the plain columnals in the lower part 

 of the proximal region, where they are less covered by the flanges. 



§ 387. The change from the proximal to the distal region is marked by the decrease 

 in diameter of the stem as a whole, and the disproportionately greater decrease in the 

 lumen, which eventually becomes quite narrow. There is also a change in the rela- 

 tions of the two classes of columnals. This consists, first, in the increased height of 

 their bodies, and in the lessening of the flanges, which soon cease to overlap and are 

 reduced from flanges to mere swollen rings. At the same time the granulation on the 

 intermediate columnals is intensified, and may form either a single ring, scarcely to be 

 distinguished from the reduced flange, or two rings with a smooth space between. In 

 respect to this ornament there is considerable specific and individual variation. 



§ 388. In the distal region the columnals appear in some cases to be elongate and 

 approximately equal in size, and the barrel shape of the upper part may be changed 

 into a more dice-box shape, partly by the development of two annular swellings on the 

 intermediate columnals as just explained, partly by the migration of the flange-swelling 

 to the upper end of the columnal and the formation of a subsidiary ring at the other 

 end. Dr Kirk appears, from his figure, to have observed an alternation in height of 

 the columnals right down the stem. 



