CARADOCIAN CYSTIDEA FROM GIR7AN. 505 



reversal of the usual process is consistent with the dominance of the vent and its 

 activities over the whole life and form of this animal. 



§ 592. Search for the ancestors of Cothurnocystis has led us to Ceratocystis and 

 Trochocystis, both of Middle Cambrian age. In outline Cothurnocystis is most like 

 Ceratocystis, but in the finely plated flexible integument it approaches Trochocystis. 

 The flattened shape of both those genera suggests a similar prostrate habit, but in the 

 mode of life there must have been diff'erences. It will be observed that the proximity 

 of intake and vent in Trochocystis is a normal pelmatozoic character, and this, no less 

 than other characters to which attention has previously been drawn (§ 252), argues 

 against an absolutely prone position, and favours the suggestion that the theca was 

 supported by some broad-leaved sea- weed. In Ceratocystis, as I would interpret it, 

 the vent retains its a-columnal position, but the intake has moved nearer to the stem- 

 attachment. If this view be correct, the respective positions are almost the converse 

 of those found in Dendrocystis. Ceratocystis, then, may well have lain on the sea- 

 floor or been only slightly raised above it ; but it is far from having all those modifica- 

 tions which adapted Cothurnocystis so admirably for such a situation, and I am unable 

 to fill in the picture of its mode of life. 



§ 593. If, in our ignorance of the position of the hydropore, we take the sagittal 

 plane as marked by the intake, the vent, and the stem -attachment, then it follows that, 

 for all the Heterostelea herein discussed, the extension-plane coincides essentially with 

 the sagittal plane. Further, there is scarcely room for doubt that Cothurnocystis lay 

 on its right side. The same was probably the case with Ce^-atocystis, in so far as it 

 approached the prostrate position. The reverse face of Trochocystis, however, the face 

 which I suppose to have been underneath, is morphologically on the left side, and this 

 is perhaps the strongest argument against deriving Cothurnocystis from Trochocystis. 

 As for Dendrocystis, the evidence is not very clear, but, on the whole, is in favour of 

 placing the brachiole and vent to the left hand when the ridged plates of the vaulted 

 region of the theca are towards the observer. I have regarded that region as upper- 

 most, on the evidence mainly of a few specimens of Dendrocystis Sedgwicki, which 

 appear to lie on the upper surface of the matrix. This evidence requires confirmation 

 by a large series of well-preserved specimens, of which the relations to the bedding- 

 plane are accurately known. Meanwhile, if my assumption be correct, it follows that 

 in Dendrocystis the morphologically left side of the theca was next the sea-floor, as 

 in Cothurnocystis. 



If the specimens of Dendrocystis scotica, G89 and G266 (§ 142, PI. II. figs. 21, 22), 

 are rightly interpreted as having the vent on this left side or reverse face of the theca, 

 they may be adduced in objection to much of the foregoing argument. But it is just 

 this that suggests an explanation for the peculiar knobs round the anal area. Were 

 they not developed to keep this area from coming into contact with the sea-floor or 

 other substratum, and so to maintain a free passage for the faecal current ? 



§ 594. It results from the preceding considerations that the common ancestor of 



