CARADOCIAN CYSTIDEA FROM GIRVAN. 



483 



§ 509. Pectinirhomb 10-14 (text-fig. 73) is approximately kite-shaped, with the 

 obtuse angle at its orad end, but is more extended on plate 14, so that it is actually 

 trapezoid. Sutural axis, 6'8 mm.; transverse axis, 8"3 mm. Length of suture 10 | 14, 

 10*2 mm.; the rhomb lies towards its orad end. In Grl42, sutural axis, 6"5 mm.; 

 transverse, 8 '2 mm,; suture-length, 9*8 mm.; the rhomb occupies about the middle 

 tract of the suture. Number of folds, ca. 28 in Gl71, ca. 25 in Grl42. In both 

 specimens, but more conspicuously in G142, the outer part of the pore-field on plate 14 

 was roofed by smooth stereom, slightly concave (PI. VI. figs. 69, 73). 



§ 510. In this rhomb in Gl71 there is some appearance as though the folds of one 

 half passed between the folds of the other half, thinning out as they do so. I have 

 noticed the appearance in other cases, and, if my interpretation 

 be correct, it points to a compression of the rhomb along its 

 sutural axis in the course of racial or individual development. 

 Phylogenetic compression is already proved by other evidence ; 

 in this species, with its wide shoulder-angle, additional onto- 

 genetic compression, not a mere recapitulation, is inherently 

 probable (cf. P. Rugeri, § 455). 



§ 511. Pectinirhomb 11-12 is trapezoidal, with a thick rounded 

 border extending far on plate 12. The rhomb lies in the middle 

 tract of the suture ; its proportions are best gathered from 

 text-fig. 79. There are about 16 folds, rather irregular, and on 

 plate 11 fanning out towards the suture, 



§ 512. The Periproct is wide. Plate 12 tapers off below plates 17 and 23, barely 

 reaching plate 18, and plate 14 abuts on plates 18 and 19, Thus 12 is separated from 

 14 by the whole width of 18 (PI, VI, figs. 70, 72). 



§ 513. The Periproctals over the greater part of the area are generally hexagonal, 

 with a diameter of about 1 mm.; but the vertical diameter is often slightly more than 

 the horizontal, and some may attain as much as 1*4 mm. Interspersed with these are 

 a few small plates of less regular shape. Near the margin the plates are smaller and 

 usually elongate (PI. VI. figs. 72, 74). 



§ 514. The Vent lies in an almost quadrangular rectal lobe, plate 3 being bent 

 downwards nearly at a right angle ; it is close to the margin, and is surrounded by 

 about 9 plates, all tapering towards the opening, but those above larger than those 

 below (PI. VI. figs. 71, 74). 



§ 515. The Ornament of the antanal face consists of strong radiate ridges, and a few 

 strong axial ridges both main and subsidiary. There are traces of concentric granulation. 



§ 516. Of the Stem, part of the proximal region is preserved in the holotype, with 

 six prominent rounded flanges separated by wide interspaces. From the bottom of the 

 first flange to the bottom of the fourth, i.e. six columnals in all, is a distance of 6 mm.; 

 the maximum height of any one of these flanges is 1'3 mm. Diameter, ca. 5 mm.; the 

 rate of tapering is very slight. 



Text-fig. 79. — Pleurocystis 

 quadrata. Pectiuirliomb 

 11-12, in the Holotype, 

 G171. X 5 diameters. 

 Cf. PI. VI. fig. 69. 



