506 DR F. A. BATHER. 



all these genera can have lain neither on its left side nor on its right, but that its 

 sagittal plane must have been vertical. Thus we are led again to that normal pelma- 

 tozoic form of indifferent structure, to which we have already worked back in seeking 

 an ancestor for Dendrocystis. 



§ 595. We turn now to the other half of the Cystid fauna of the Starfish Bed, 

 namely, the elements contributed by the Glyptocystidea. Like the Heterostelea, 

 though by no means to so great an extent, this Superfamily includes several genera 

 that have been modified for a partially eleutherozoic existence, and among them several 

 seem to have assumed, or at least to have approached, a prostrate habit. Referring to 

 the list of genera (§ 281), we note among the Echinoencrinidae such forms as Echino- 

 encrinus and Erinocystis, which apparently rested the theca on the sea-floor, although 

 geomalic growth did not supervene (see Kirk, 1911, p. 19). The compressed form 

 characteristic of that growth is, however, shown in the allied genera Schizocystis and 

 Glaphy7'ocystis (G. compressa). The Callocystidae seem to have retained the pelma- 

 tozoic attitude, but Trimerocystis may have approached, and Pseudocrinus probably 

 attained, a position parallel to the sea-floor, though not in actual contact therewith. 

 I see no better explanation of the extremely flattened form of Pseudocrinus and 

 the reduction of its subvective grooves, than that it lived in the manner I have 

 imagined for Dendrocystis. 



§ 596. It is, however, in Pleurocystis, a representative of the third Family, the 

 Cheirocrinidae, that the connection of structural modifications with a prostrate position 

 is most obvious. But, as Dr Jaekel has pointed out, the path along which adaptation 

 proceeded was here quite diff"erent from that which it followed in the other Families. 

 In them the periproct remains relatively small, but in Pleurocystis its extreme enlarge- 

 ment constitutes an essential feature. In regard to this feature the Cheirocrinidae are 

 divisible into two groups : Glyptocystis and Cystohlastus as opposed to Cheirocrinus 

 and Pleurocystis. 



§ 597. In Glyptocystis, as Dr Kirk insists (1911, p. 17), "the negative evidence 

 of the lack of special adaptation to a prostrate mode of life points strongly to the 

 conclusion that in this genus an erect position was constantly maintained." In Cysto- 

 hlastus the strongly marked pentamerism of the subvective system bears even more 

 forcible witness to an upright habit. 



§ 598. In Cheirocrinus, on the other hand, it is difficult not to recognise the early 

 stages of that periproctal enlargement and thecal flattening which culminated in Pleuro- 

 cystis. Yet the matter is by no means so simple. It is impossible to trace a gradual 

 increase in the size of the periproct, in the number of periproctals, or in the flattening 

 of the theca, as we follow the history of Cheirocrinus from the bottom to the top of 

 the Ordovician. The latest species known to us, C. constrictus, is not one whit more 

 modified in this direction than is C. sculptus from the base of the Vaginatenkalk. But, 

 since Pleurocystis is known from the lower beds of the Trenton Limestone, the species 

 of Cheirocrinus from the later half of the Ordovician do not come into its ancestry ; 



