CARADOCIAN CYSTIDEA FROM GIRVAN. 401 



There are also some more incomplete specimens placed here provisionally, viz. : 

 G7, G37, G60-63, G202, G215, G225. 



G64 preserves traces of the stereom in stem, frame, and integument. All the other 

 specimens are in the form of imprints, and about half retain their counterparts. The 

 description is based on squeezes taken from these. 



§ 174. Description. — The Thecal Frame, as viewed from the obverse side, forms 

 an outer border of irregular boot-shape, as shown in text-fig. 15 (PL III. figs. 27, 31). 

 In this view, the toe of the boot, with its long spine, is always to the left of the ob- 

 server, the leg with its tag and tongue processes are to his right. The hollow of the 

 sole lies at the attachment of the stem, the ball of the foot being to the left and the 

 heel to the right. At the heel the frame bends upwards sharply, at a right angle to the 

 sole, or even less ; at the ball of the foot, either it bends perceptibly but at an obtuse 

 angle {e.g. G26), or curves round gently towards the toe [e.g. G45). The reverse view 

 (text-fig. 14) shows another element of the frame, the Strut, continuing the fore-side 

 of the leg down to the sole, which it joins on the heel-side of the stem (G42, G40, 

 G26, G6, G41 ; for the last three, see PI. III. figs. 26, 28, 37). 



§ 175. The frame is built of several elements — Marginalia, and, since the sutures 

 between these seem to correspond precisely on the two faces, and there is no sign of 

 any median joint in the extension-plane, we infer that they are arranged in single series. 

 The evidence of the sutures, gleaned from all available specimens, shows that the 

 positions of the marginals are almost constant, and that their number is normally 

 twelve, occasionally thirteen. Their relations are more conveniently studied on the 

 reverse face ; therefore, taking that face and beginning at the top of the leg, below the 

 tag, we number the marginals all round the theca, in a contrasolar direction. 



§ 176. Marginal 1 is a short straight piece, immediately below the tag, which it helps 

 to support. 



§ 177. Marginal 2 is rather longer than 1, is straight or slightly bent, with the 

 concave curve outside. About the middle of its length it bears a knob (G22, PL III. 

 fig. 30). 



§ 178. Marginal 3 is L-shaped and forms the heel. The vertical limb of the L is 

 rather short, the angle itself is usually broadened and thickened, the horizontal limb is 

 usually longer and narrower. In addition to, or distinct from the thickening of the 

 angle, there is a knob at the angle end of the horizontal limb (see Gl2, Gl3, G33 ; 

 also PL III. figs. 28, 37). 



§ 179. Occasionally a short knob-bearing piece, 3a, intervenes between marginals | ^^^n-^tX^^ 



3 and 4, and in that case the horizontal limb of the L is correspondingly shortened 22.1, ■>- p-.TS; 

 (see G33 and G41, PL III. fig. 37). I '^^"4I^^f'-. 



§ 180. The middle line of the stem coincides with the suture between marginals 



4 and 5, which embrace the stem-lumen by their apposed ends (PL III. figs. 31, 36). 

 Marginal 4 sends a process upward, on this reverse side, to form the lower part of 

 the strut. 



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