56 Br. F. A. Bather — Studies in Edrioasteroidea. 



most pronounced stem, and that manifests in its thecal structure 

 the most pronounced reaction to such a mode of fixation, still it 

 differs greatly from Pyrgocystis in the crinoid-like nature of that 

 stem, in the solidity of the theca, and in the Edrioasterid character of 

 the subvective skeleton. Cyathocystis again, especially in its more 

 elongate individuals, presents a subvective system of straight grooves 

 mounted on a turret of much the same proportions as in P. sardesoni. 

 But the turret in Cyathocystis is a solid tube, and the structure of the 

 whole oral face is totally different. 



Among the Agelacrinida? are some genera that approach Pyrgocystis. 

 Cystaster granulatus as defined by J. Hall (October, 1871), from which 

 Jaekel has attempted to separate some specimens as Thecocystis sacculus 

 (1899), has straight rays surmounting an elevated sac-like body 

 composed of minute plates or granules. In Memicystis also the rays 

 are straight, and Hall (October, 1871) specially mentions that 

 H. parasitica and H. stellata have " the sides more abruptly elevated 

 than the ordinary forms of Agelacrinus". The theca in Semicystis is 

 composed of minute squamif'orm plates. Agelacrinus pilens is said 

 by Hall to have a " globular bell-shaped" theca when well preserved, 

 and he figures a specimen in which many of the interradial plates 

 " have a rounded node near the centre", possibly a spiniferous tubercle. 

 In this species, as in all "Agelacrinus", the rays are curved. 



The occurrence of spines in Pelmatozoa was probably more common 

 than is generally supposed, but their preservation is certainly rare. 

 Had specimen a of P. sardesoni not been found, nobody would have 

 supposed thatvany species of Pyrgocystis bore spines; and the fact 

 that they are not known in the other species is no proof that they 

 were not present. No stress should be laid on the appearance of 

 vertical rods on the cup-plates of P. grayce, since they may be merely 

 strengthening ridges. Meek (1873, Rep. Geol. Surv. Ohio, vol. i, 

 pt. 2, p. 56) mentions ridges on the inner surface of marginal plates 

 in an Agelacrinus cincinnatiensis (?). 



Bionomics of Pyrgocystis. — The three formations from which 

 specimens of this genus are known are of different lithological 

 character. 



P. sardesoni is found in a thin-bedded limestone, intercalated in soft 

 green shales, and composed of numerous small fossils and fragments, 

 but mainly of bryozoan skeletons, especially of branching bifoliate 

 forms. This seems to suggest pure, relatively shallow water, crowded 

 with animal life. The skeletons afforded a firm base of attachment 

 for the Edrioasteroid's turret, which was under no necessity to be 

 lofty since there was abundance of food and of aerating currents. 

 There is no indication that the turret was fixed in other than a vertical 

 position. The spines might conceivably be ascribed to some innate 

 acanthogenous agency, or (less mystically) to an excess of lime in the 

 water; but, whatever the contributory causes, we may suppose that 

 the longer spines on the oral face were useful as a defence against 

 worms and other predatory foes, while the spinules on the turret 

 may have protected the soft stroma from parasitic organisms. 



P. grayce is preserved in a sandstone from which, as generally 

 found, the calcareous constituents have been leached. This also was 



