CARADOCIAN CYSTIDEA FROM GIRVAN. 487 



about half the height of the others, but, to judge from the imprint showing the 

 interior, more proximal columnals stretched up under the basal circlet (cf. § 526). 



S 534. Comparison with other Species. — From P. Rugeri, P. anglica, and 

 P. procera, this species is readily distinguished by the greater development of its 

 pectinirhombs and in particular by their elongation. In a lesser degree the same 

 characters serve still to distinguish it from P. quadrata, which approaches it in 

 general shape (see § 519). In the elongation of pectinirhomb 10-14 and in the 

 shoulder-angle P. gihha agrees with P. foriolus ; the periproctals also may have had 

 about the same diameter in the two species. 



P. foriolus, however, has a very different appearance, owing to its more ovate 

 shape, without the sharp hump of P. gihha, and to its more marked anal lobe ; in it 

 pectinirhomb 11-12 is wide rather than elongate, and does not occupy so large a tract 

 of the suture ; the number of rhomb-folds is considerably less. Since the specimens 

 of P. gihha are larger than all those referred to P. foriolus, it is possible that the two 

 forms are merely different growth-stages of the same species, and that P. gihha stands 

 to P. foriolus in the same relation as the holotype and paratype of P. quadrata were 

 thought possibly to stand to the smaller specimens provisionally associated with them. 

 In the present case, however, this suggestion seems to me to be contradicted by the 

 more developed ornament and anal lobe of P. foriolus. 



Pleurocystis foriolus n. sp. 

 (Plate VI. figs. 75-81. Text-figs. 74, 75, 80.) 



§ 535. Diagnosis. — A Pleurocystis with subovate theca ; shoulder-angle, 90°-100° ; 

 ratio, width : height, of theca, ca. 1 ; of plate 5, ca. 1 ; plate 3 descends to third or fourth 

 flange, forming one side of a clearly marked rectal channel. Pectinirhombs all present, 

 concave, with sharp border ; 1-5 square to trapezoid, 10 to 12 folds ; 10-14, kite-shaped, 

 18 to 25 folds, sutural axis : transverse axis : : 100 : ca. 50 to 96 (?) ; 11-12, trapezoid, 

 tends to be widened, 10 to 11 folds. Plates 12 and 14 separated. Diameter of peri- 

 proctals usually "4 mm. ; occasionally a few may reach '9 mm. Ornament of antanal 

 face, granules in concentric rows, running into fine but well-marked rhombic and 

 radiate ridges, variable. Stem with granules in transverse rows and with some 

 tendency to a vertical arrangement. 



§ 536. Locality. — Thraive Glen, Girvan. 



§ 537. Horizon, — The Starfish Bed in the Drummuck Group (§ 6). 



§538. Material.— G 189, taken as holotype (PI. VI. figs. 79-80), consists of 

 imprints of the anal and antanal faces with about 29 mm. of the stem. Gl35 

 (PI. VI. figs. 75, 76) is a somewhat similar specimen, less complete. G213 (PL VI. 

 figs. 77-78) yields both imprints, but is imperfect in the proximal region, and the 

 plates are shifted. Gl70 and G223 (PI. VI. fig. 81) may be described as internal casts 

 which give by squeezes an internal view of the antanal face, with traces of the anal 



TRANS. ROY. SOC. EDIN., VOL. XLIX. PART II. (NO. 6). 64 



