CARADOCIAN CYSTIDEA FROM GIRVAN. 373 



concerning other portions of the skeleton rest solely on the evidence of the other 

 species. The previous account by Barrande and the few statements added by Jaekel 

 have been carefully checked. See generally text-figs. 6-9. 



S 45. Theca, Form. — When viewed from either face of the extension-plane, the 

 outline is roughly that of an isosceles triangle with truncate apex. The ratio of width 

 to height varies according to species from '9 to r5. The stem is attached to the base 

 of the triangle, and there is a tendency for the lower angles to form rounded lobes 

 which may project below the stem-attachment. Thus is produced a reversed heart 

 shape. The increase of these lobes makes the sides of the triangle concave. This is 

 particularly the case on the antibrachial ( = antanal) side, where the lobe is often more 

 pointed and more prominent. On the brachial ( = anal) side, the outline is straighter 

 or even tends to convex, and the lobe as a rule is not so prominent. This is called the 

 brachial side because it is on this^side that the appendage springs from the truncation of 

 the triangle; it may be called the anal side because, in D. scotica at any rate, there is good 

 evidence for the presence of a vent between the stem and the lower angle on this side. 



§ 46. A further modification of the truncate isosceles triangle consists in the pro- 

 longation of the antibrachial angle of the truncation as a lobe counterbalancing the 

 appendage. In the Bohemian species this is little more than a rounded hump, but in 

 D. scotica it may reach a length of over 17 mm., while the plates composing it assume 

 a hexagonal outline and a definite arrangement. 



§ 47. The tenuity of the plates might lead to the supposition that the compression 

 of the theca was a mere consequence of fossilisation. Barrande himself conceived of 

 the theca as ovoid or spheroidal, and even Jaekel describes it as " beutelformiger, 

 wenig comprimirter." But that considerable compression characterised the theca in 

 life seems to follow from the fact that all except one or two of the specimens in the 

 rather large series that I have examined seem to be compressed in the same, or at any 

 rate approximately the same, plane. The compression and shearing due to fossilisation 

 is greatest in the black micaceous shales of Zahorzan ; and this, combined with the 

 number of fossils and fragments scattered through the rock, renders the true outline 

 hard to see in D. Sedgwicki. But the specimens preserved in quartzite or sandstone 

 (Z>. Barrandei and 1). scotica) retain, in many cases, a definite form and outline, 

 clearly very little disturbed by post-mortem pressure. The greatest thickness for 

 which these aff"ord any evidence is 8 mm. in D. Barrandei (E 16027), and this is 

 only on the assumption that one face was not concave. Probably no theca of Dendro- 

 cystis was more than 1 cm. from face to face ; and, at a rough estimate, the thickness 

 did not as a rule exceed one-third the greatest width. 



§ 48. The compression, however, was not so great as to produce any notable modi- 

 fication of the plates at the margins. In the earlier species one face seems to have 

 passed gently round to the other, without any sharp demarcation between the two or any 

 bending of individual plates ; in D. scotica, however, the adcolumnal margin of the lobes, 

 especially the antanal lobe, often seems to be stiff"ened by irregular bent marginals. 



