CARADOCIAN CYSTIDEA FROM GIRVAN. 419 



S 248. The restoration of Trochocystis published by me in 1900 (p. 50, f. xi.), 

 represented the oro-anal region quite incorrectly. This is because it was based on 

 Barrande's fio-ures, which are very difficult of comprehension. The kind help of 

 the Bohemian Museum has now enabled me to see my error, and to recognise that 

 Dr Jakkel's representation of this region (1901, p. 663, f. l) is essentially correct, though 

 I am not fully prepared to accept all the details in his restoration in f 6 A, p. 670. 



§ 249. Neither in the Bohemian specimens nor in other specimens preserved in the 

 British Museum can I distinguish the arrangement of the integumentary or somatic 

 plates near the vent. Specimens of the allied Mitrocystis, however (Brit. Mus. 

 E7517, E 16070), prove that the structure depicted in Barrande, pi. 4, ff. 1, 8, 14, 

 and others, has nothing to do with a folded madreporite as I hesitatingly suggested 

 (1900), but is merely the imprint of small elongate plates converging towards a 

 sphinctered anus, precisely as in Cothurnocystis. It is this fact, already recognised 

 by Jaekel, which decides the position, if not the structure, of the vent in 

 Trochocystis. 



At this point I would ask the reader to study carefully text-figs. 29-32, with their 

 accompanying description (see next page). He will then be in a better position to 

 appreciate the following discussion. 



§ 250. The main points in which Trochocystis differs from Cothurnocystis are : 

 (l) the general outline of the theca, which is more nearly circular, or sometimes sub- 

 triangular, with the apex of the triangle at the insertion of the stem, as in those forms 

 which Haeckel (1896) has called Trigonocystis. This rounder outline and less lateral 

 extension of the theca shows that Trochocystis, even though it may have departed 

 somewhat from the normal pelmatozoic habit, had not become modified thereby so 

 much as Cothurnocystis. (2) The position of both intake and vent at the pole of the 

 theca opposed to the stem. This also proves that Trochocystis was nearer the normal 

 Pelmatozoan. (3) The existence of only a single mouth, to which lead two grooves ; 

 and the position of these structures not on the plating, but on the marginals. (4) The 

 polymerism, usually dimerism, of the stem, which has no regions diff"erentiated as those 

 of Cothurnocystis. (5) The morphological relations of the reverse face, which appears 

 to be to the left of the sagittal plane. 



§ 251. Although Trochocystis retains several traces of a more normal pelmatozoan 

 ancestry, its departure from an upright habit of life is already manifested in the short 

 tapering stem and the flattening of the theca. That one face of the theca was directed 

 towards the sea-floor, the other away from it, is proved by the excavation of the 

 marginals for intake and vent on one face only, and it is noticed that the plating on 

 that face is finer than on the reverse. 



§ 252. On the other hand, it is hard to imagine that Trochocystis lay prone on a 

 sea-floor covered with sand such as that composing the rocks in which its remains are 

 found, for the supposed food-grooves would then have been very slightly raised above 

 the sand and would frequently have been choked by it. Dr Jaekel seems to 



