CARADOCIAN CYSTIDEA FROM GIRVAN. 



423 



were very different from those of Cothurnocystis ; and, taking these facts in connection 

 with the different morphological relations of the sagittal plane, we must conclude that 

 Cothurnocystis was not directly descended from Trochocystis. 



\ 255. But there is in the Middle Cambrian of Bohemia another form, which Dr 

 Jaekel (1901, p. 667) has referred to the Heterostelea, under the name Ceratocystis 

 Perneri. Seeing that only a few lines of description and two rough diagrams have 

 been published, it is fortunate that the British Museum has been able to acquire four 

 specimens (E 1607 i-E 16074), comprising the remains of about ten individuals. 

 Examination of these brings out many striking resemblances to Cothurnocystis. 



§ 256. Ceratocystis (text-figs. 33, 34) has a compressed theca with a relatively 

 slender stem. The outline of the theca is unsymmetrical, and, though not so markedly 



veut 



vent 



tongue 



toe-spine 



ball of foot 



heel 



heel 



Fig. 33. 



Fig. 34. 



Text-figs. 33, 34. — Ceratocystis Perneri. Diagrams based on Brit. Mus. E 16072-3-4. Natural size. 



Fig. 33. — Reverse face, showing the stem in section, with the fluted infilling. The precise outlines of the plates are not qnite 



certain ; there is not much difference between this and Jaekel's figure (1901, p. 667, f. 4 B), but the material at hand does 



not enable every point to be controlled, especially the region near the stem. 

 Fk;. 34.— Obverse face, showing the exterior of the stem. The lateial processes or appendages of the distal region of the stem 



are not indicated, since it is not clear that they are other than the displaced elements of the columnals themselves. The 



positions of the subvective grooves are taken mainly from El 6074 (cf. text-fig. 35). 

 The names applied to dilferent parts are based on the homology with Cothiirnoeystis. The vertically ascending limb of 



the triradiate ridge approximately corresponds in position to the strut. 



boot-shaped as that of Cothurnocystis, has regions corresponding to the leg, the heel, 

 and the foot-half. A distinct frame is not obvious, since the central plates of each face 

 are few and not markedly smaller than the marginals. The marginals, however, though 

 of irregular shape, are more massive than the central plates, and appear to have been 

 bent round the edges so as to have a V-shaped or U-shaped section. At the top of 

 the leg the marginals are extended into two processes corresponding in position to the 

 tag and tongue of Cothurnocystis, but not separated from the body of the marginals 

 by any suture or articulation. The marginal at the heel is triangular and greatly 

 thickened ; the same is the case with the marginal at the other lower angle of the 

 theca, corresponding in position, not to the toe of Cothurnocystis, but to the knob- 

 TRANS. ROY. SOC. EDIN., VOL. XLIX. PART II. (NO. 6). 56 



