202 F. A. Bather — Studies in Edrioasteroidea — //. 



made by some system of organs that had undergone division into 

 five, and had the pentameres interradial. The stomach, gonads of 

 Echinoid type, and water-vascular system seem therefore to be 

 excluded, vs^hile the chambered organ of crinoids is forcibly suggested. 

 It is clear that the frame of 11 large plates gave some rigidity 

 to the base, while within this the membrane was very flexible, 

 capable of extension and contraction. These arrangements may 

 have served a twofold purpose. If the theca were compressed by 

 any external agent, the soft parts and the fluid contents of the coelom 

 would have been squeezed out into this extensile sac, which thus 

 acted as a safety-valve. Or the vertical mesenteries of the ccelom 

 may have been attached to it, and, developing muscles as in 

 Echinothuridee, may have been able to withdraw it within the 

 frame ; if the outer edge of the theca were closely apposed to the 

 ground, the eifect of this would be to create a vacuum and so hold 

 the creature in its position like a limpet. In the dead animal, when 

 the tissues shrank, the aboral membrane was naturally pulled 

 upwards, and at the same time other parts of the theca were 

 pulled inwards. Hence arose the stretching of the membrane over 

 the internal (? chambered) organs, in the same way as the wrinkled 

 depression already noticed in the posterior interradius. Jaekel's 

 opinion that the central area of the abactinal surface was fixed 

 {aufgeioaclisen) on a not quite hard bottom, does not seem in accord 

 with the shape of the theca or with the sharp definition of these 

 five lobes. 



Other Specimens. 



The various stem -fragments ascribed by Forbes to this species 

 can, as already noted by Salter and Etheridge, have had nothing 

 to do with it. They were found in the same stone wall, but not 

 in juxtaposition with the theca. Neither can the 4-rayed impression 

 so badly represented in Forbes' pi. xxiii, fig. 14, be regarded as 

 " the impression of the base, probably of a similar cystidean." 



There was found, however, one other fragment, which I am 

 unable to fit on to the type-specimen, and therefore regard as 

 indicating the existence of another individual. It is an impression 

 of part of an ambulacrum, 16 mm. long, and of portions of the 

 adjacent interambulacra, one of which shows very clearly the 

 markings produced by the ornament of the plates (Pi. X, Fig. 3). 



No other specimen of this species is known. 



Systematic Position. 



Forbes called this species Agelacrinites Bucliianus, comparing it 

 first with the specimen discovered by Bigsby, and described and 

 figured by G. B. Sowerby in the Zoological Journal (vol. ii, 

 pp. 318-320, pi. xi, fig. 5, 1826), and afterwards ndLmedi Agelacrinites 

 DicTcsoni by E. Billings (1856) ; and secondly with Agelacrinites 

 Jiamiltonensis, Vanuxem, which is the type of the genus. Forbes 

 considered it as a Cystid, allied to the Echinoidea. 



Salter (op. cit., p. 290), while retaining the species in Agelacrinus, 

 considered it as more allied to the species described by Billings as 



