CARADOCIAN CYSTIDEA FEOM GIRVAN. 407 



§ 202. The conical ossicle of the Middle region serves to reduce the diameter from 

 that of the wide rings to that of the narrow columnals. The diameter of the lumen 

 probably undergoes a corresponding reduction. In most specimens this ossicle seems 

 to have been rather solid and massive. In G34, however, where it has apparently 

 been weathered and broken open, there are traces of four originally component rings, 

 and of a vertically fluted structure in the lumen (cf. G23, G26, G54, also G27, 

 PI. III. fig. 32). 



i^ 203. The Distal region of the stem has a length not less than three times that 

 of the proximal and middle regions together. Its diameter at the proximal end is equal 

 to the diameter at the narrower distal end of the reducing piece. Thence the stem 

 tapers very gradually and quite equably as far as it can be traced in any specimen. 

 Thus in G267 (PI. III. fig. 29) the diameter lessens from ri mm. to 0"3 mm. in a 

 length of 26*5 mm. This region is composed of circular columnals with a height from 

 one-half to two-thirds the diameter. Occasional slight irregularities may occur (e.g. 

 G267). The sutures do not appear to be crenelate, and the joint-faces seem to have 

 been smooth and slightly concave (G25). Weathering of the stem occasionally exposes 

 a vertical fluting as noticed above (G27, PI. III. fig. 32 ; G267, G268). 



§ 204. The greatest total length of stem observed is about 50 mm. in Gl8, where 

 the width of the theca, toe to back of leg, is about 28 mm. This is relatively as well 

 as absolutely the greatest length observed ; therefore it is not likely that the stem 

 exceeded twice the width of the theca. 



§ 205. There is no trace of a root or of any means of attachment, and it is probable 

 that the stem, after its gradual tapering, was rounded off" abruptly. It may, of course, 

 have been fixed to a root at an earlier stage of growth ; but it is not likely that there 

 was any temporary attachment by the coiling of the end of the stem, for the whole 

 distal region of the stem is remarkably straight in the fossils, and, as one would expect 

 from the joint-faces, must have been greatly lacking in flexibility. 



§ 206. The stem, however, is not always found lying straight at right angles to 

 the sole of the boot-shaped theca, but is frequently deflected, now towards the toe 

 as in G267 (PI. III. fig. 29), now towards the heel as in G268, G269, and G50 

 (PI. III. fig. 27). The curvature producing this is confined to the proximal region 

 of the stem, where the peculiar articulation of the dimeres permitted enormous 

 flexibility. 



But whatever the angle that it forms with the sole, the stem always lies in the 

 plane of extension ; it is never bent under or over the theca. This fact suggests that 

 the flexibility of the proximal region was so restricted as to permit movement only in 

 that plane. 



§ 207. Measurements of Cothurnocystis Elizas in millimetres. In each case the 

 figures give the outside or "over all" measurement, as taken from a squeeze. For 

 stem measurements, see also § 203. 



TRANS. ROY. SOC. EDIN., VOL. XLIX. PART II. (NO. 6). 54 



