422 DR F. A. BATHER. 



represent cover-plates in his, no doubt purposely, sketchy reconstruction (1901, p. 670, 

 f. 6 A), but offers no evidence. That these elements did exist is proved for T. 

 hohemica by the original of Barrande, pi. iii. if. 23-25, and for T. Barrandei by 

 Brit. Mus. E 7694 and E 7696 ; but they are very small, and by no means so regular 

 as indicated in Jaekel's diagram (f 1). Further, there is no evidence for such free 

 appendages (? brachioles) as are adumbrated in Jaekel's f 6 A. If, then, the subvective 

 system consisted of simple marginal grooves, an absolutely prone habit is inconceivable. 

 Possibly the animal attached its theca, by a sucking action of the reverse face, to 

 some broad-leaved sea-weed, of which, naturally, no traces remain. 



^ 253. If, now, we suppose that Trochocystis did in time come to lie flat on the sea- 

 fioor, certain modifications would seem necessitated. Of these the most urgent would 

 be the passage of the subvective system to a position where it would not inevitably 

 be choked with sand ; in short, it would have to move on to the obverse face. Some 

 attempt at this, probably preceding the actual removal to the sea-floor, is manifested 

 by the species found in Herault (text-figs. 31, 32), in that the marginals are flattened 

 and extended on the reverse face and excavated with a broad groove on the obverse 

 slope. The actual subvective groove arises as a depression in this broad groove, and 

 so is visible in the obverse view. But for the groove to have passed over the edge of 

 the marginals on to the small somatic plates of the obverse face would not have been 

 a natural transition. The actual intake, however, was already bounded on one side by 

 somatic plates, and the same pressure of circumstances would tend to drive it further 

 into the somatic area of the obverse face. As the intake passed away from the margin, 

 the ciliated grooves would, of course, follow it, and in this way the transition might be 

 more naturally effected. At first there might be some inward bending of the thecal 

 frame in the oral region, but eventually the mouth and groove might break right away 

 from the marginals. 



The modification next in order .of importance would be some means of protecting 

 the subvective system from the anal stream, and this would be brought about by 

 increasing the distance between vent and intake, or by controlling the direction of the 

 out-current, or by both methods conjoined. 



Such modifications as the further protection of the subvective system by distinct 

 rims, or the raising of the obverse face above the sandy floor, would all be to the 

 advantage of the animal. 



§ 254. It becomes plain, therefore, that any modifications of Trochocystis likely to 

 occur in connection with such a change of life would all be in the direction of 

 Cothurnocystis. But when we attempt to control these speculations by reference to 

 the actual fossils of intervening age, we find that the course of evolution, so far as it 

 has hitherto been traced, followed a different path. For, in the Lower Ordovician, 

 Trochocystis is succeeded by such genera as Mitrocystis, which in the Upper 

 Ordovician, Silurian, and Lower Devonian, give place to the typical Anomalocystidae. 

 Whatever the habits of life that induced that evolution, it is at least certain that they 



