CARADOCIAN CYSTIDEA FliOM GTRVAN. 



425 



§ 259. In drawing the above comparison I have not wished to claim Ceratocystis as 

 a direct ancestor of Cothurnocystis. Starting from Trochocystis, the trend of evolution 

 with which we have hitherto been familiar is towards a more heavily-plated form, and 

 Ceratocystis may have arisen from some intermediate link in that series as yet unknown 

 to us. The similarity of the curiously ornamented stereom in Ceratocystis to that in 

 both species of Trochocystis, at any rate, suggests some relationship. Where Cerato- 

 cystis departs from the main line of evolution is in the assumption of asymmetry in 

 place of proceeding to greater bilateral symmetry. The change of habit that induced 

 this divergence must have been the same as that which affected the later Cothurnocystis. 



Text-fig. 35. — Ceratocystis Perneri. 



Photograph from a squeeze of an imprint on Brit. Mus. E 16074, from the Paradoxides beds at Slapnitzer Miihle, 

 Skrej, Bohemia, x 3 diameters. 



The projection homologous with the toe-spine of Cothurnocystis is in the middle line, at the top of the figure. The 

 curved notch is seen to the right of it, and beyond this the specimen is cut off. On the extreme left of the figure is the 

 stout, pointed marginal corresponding to the ball of the foot in Cothurnocystis. From this marginal a ridge runs to a point 

 above the stem, whence a ridge passes upwards to the front of the leg, and another downwards to the heel ; but these two 

 ridges are not clearly shown. Along the inner edge of this same marginal is a series of rounded notches, and a similar 

 series follows the lower edge of the adjacent plate that bears the above-mentioned ridge. The latter series coincides with a 

 similar series on the edge of the underlying marginal. Thus was formed a continuous series of elliptical openings along 

 the suture from the pointed marginal to the stem. 



The coarse meshwork of the plates, with their rugose surface, is clearly shown, as is also the structure of the stem. 



But it does not seem altogether reasonable to suppose that, having once lost the flexible, 

 finely-plated integument, this particular line of descent should have redeveloped it. 

 Of course, a flexible integument may be developed in connection with the vent, as we 

 shall see under Pleurocystis ; but in Cothurnocystis the small plates cover the reverse 

 as well as the obverse face, and the general tendency in such forms is for the plating 

 of the reverse face, at any rate, to become heavier. Therefore Cothurnocystis, though 

 later in time than Ceratocystis, may have arisen from some relation of Trochocystis that 

 retained the frame and flexible integument. 



