CARADOCIAN CYSTIDEA FROM GIRVAN. 485 



of the upper rhombs seem to distinguish the species well enough from the other 

 British species in which rhomb 10-14 is widened. 



Some of the doubtful specimens do not differ greatly from some specimens that 

 are referred to P. foriolus, and they may belong to that species, 



Pleurocystis gihha n. sp. 

 (PI. VI. figs. 64-68. Text-fig. 71.) 



§ 520. Diagnosis. — A Pleurocystis with markedly subquadrate theca, sharply bent 

 at the umbo of plate 12 ; shoulder-angle, ca. 95°; ratio, width : height, of theca, ca. '86 ; 

 of plate 5, r09 ; plate 3 descends to below third flange. Pectinirhombs all present, 

 concave, with sharp, steep border; 1-5 square to rhomboid, widened, 15 to 20 folds; 

 10-14, approximately kite-shaped, about 50 folds, sutural axis : transverse axis 

 :: 100 : 80; 11-12 elongate, about 25 folds. Plate 12 separated from, or only just 

 touching, 14. Diameter of periproctals, ca. "5 mm,, a few possibly larger. Ornament 

 of antanal face, fine granules in concentric rows and forming fine, faint, rhomb-ridges 

 and broader, obscurer, radiate ridges. Stem proximally with bold flanges bearing 

 granules in vertical series. 



§ 521. Locality. — Thraive Glen, Grirvan. 



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



§523. Material. — Gl72, holotype (PI. VI. figs. 66-68), a specimen preserving 

 the imprint of the antanal face, the internal cast yielding an imprint of the interior of 

 the antanal face, and an imprint of the anal face. G 177, paratype (PI. VI. figs. 64, 65), 

 imprint of the antanal face and of part of the anal face. Both specimens are in- 

 complete and have the plates shifted, but since they are obviously of the same species 

 they supplement each other. 



§ 524. Description of Specimens. — The right side of the antanal face ascends 

 in an almost straight line parallel to the main axis, from the umbo of plate 7 to 

 the umbo of plate 12, and then bends sharply towards the oral pole. The left 

 side ascends in more curved line from plate 8 to plate 14, with the chord of the 

 curve parallel to the main axis ; it then curves round, without a very sharp bend, 

 towards the oral pole. Thus the general shape is quadrate. Owing to the sharp 

 bend on plate 12, giving the outline that hunch-backed appearance which has suggested 

 the name, the plates in that region are disturbed by pressure, especially in Gl72, and 

 the shoulder-angle is exaggerated; in estimating it at 95°, I have made considerable 

 allowance for that fact. 



Thecal width in Gl72, 26 mm. ; height, 30 mm. ; ratio, '86. In Gl77, the corre- 

 sponding measurements are uncertain, but may be estimated as 22 mm., 26*5 mm., and 

 ratio -83. 



§ 525. Plate 5 is irregularly hexagonal, being considerably extended to the left ; 

 its measurements in Gl72 are: width, ca, 12 mm.; height, 11 mm.; ratio, 1-09. 



