II. Edrioaster Buchianus. 



1900 * 201 



however, was repeated in modified language by Etheridge (op. cit., 

 p. 482), and it may therefore be considered that two palaeontologists 

 of repute were, as I am, unable to see the diplopores described by 

 Forbes and drawn by C. R. Bone. 



The most important features of the abactinal membrane are 

 5 prominences or evaginations of it. They form a fairly regular 

 pentagon, each corresponding to an interradius, but they bear no 

 evident relation to the angles of the central area. Each evagination 

 has a rounded U-shaped margin, the apex directed away from the 

 thecal axis. The raised margins are not absolutely continuous. 

 Within the margins the membrane is again depressed, and just in 

 the middle, about the actiual pole, it cannot be traced in either cast 

 or impression. Possibly it was here very thin and contained no 

 imbricating calcareous plates. There is, however, no evidence, that 

 there was either a stem-attachment, as Forbes thought, or any 

 opening from the gut or the body-cavity to the exterior. 



These evaginations of the aboral membrane, alluded to by Forbes 

 as "prominences" (p. 522) and "peduncles" (p. 538), received another 

 explanation from Wyville Thomson. 1 He writes (p. 110): "In the 

 specimen of A gel. Buchianus (Forbes), in Jermyn Street, there is 

 a rudely pentagonal stamp on the apical surface, which is probably 

 the impression of the wide base of a pyramid of jaws like that of 

 Echinocystites, on the inside of the coriaceous integument." This 

 is very incorrectly expressed, and were it not for some remarks by 

 Salter (op. cit., pp. 290, 291), would be unintelligible. By "the 

 pentagonal stamp," Thomson seems to have meant, as Salter says, 

 "the five indentations figured by Forbes, rig. 6," i.e. indentations 

 not on the apical surface, but on the impression of that surface, 

 and therefore not really indentations but prominences. Salter, 

 attempting to improve on Thomson, speaks of them as " large buccal 

 teeth . . . which must, I suppose, represent the ' lantern ' in 

 the Echinus." 



To this interpretation of the prominences there are objections 

 of three distinct kinds. First, the specimen, though it shows traces 

 of the circumoral ring plainly enough, shows within or below this 

 ring no traces whatever of jaws or teeth. Secondly, the buccal 

 armature of Palmodiscus would not make an impression of this 

 nature or in this position, while that of Echinocystis is not large 

 enough to make any such impression at all. 3 Thirdly, it is incon- 

 ceivable that an Echinoderm of distinctly pelmatozoan type, with 

 mouth upturned and with deep subvective grooves, can have had any 

 use for stout biting jaws ; even if descended from a gnathostomatous 

 Echinoid (a quite absurd supposition) it would have lost its jaws 

 before attaining its present habit and structure. 



The meaning of these five extrusions of the aboral membrane 

 must be sought among other Pelmatozoa. They must have been 



1 C. Wyville Thomson, "On a new Palaeozoic Group of Echinodermata " : 

 Edinburgh New Phil. Journ., n.s., vol. xiii, pp. 106-117, pis. iii, iv ; Jan., 1861. 



3 See W. J. Sollas, " On Silurian Echinoidea and Ophiuroidea " : Quart. Jouru. 

 Geol. Soc, vol. lv, pp. 692-715 ; Nov., 1899. 



B 



