THE BLASTOIDEA 



89 



b.r.J 



late central area, with a median ridge, separated from the ambulacra by 

 transversely grooved areas meeting adorally. Some specimens show 

 appearances of sutures between these areas, especially at the aboral end. 

 Hence Lyon (1857), from the study of many hundred specimens, and 

 Billings (1870) concluded that these interambulacra also contained three 

 plates, as a rule fused together. Roemer and Etheridge & Carpenter, 

 however, believed that there was only one plate, and that the markings 

 were merely ornament. The am- 

 bulacra are narrow ; their struc- 

 ture is shown in Figs. X. 2 and 

 XL They dip down to the mouth 

 underneath a roof of strong plates. 

 The hydrospires are pendent, their 

 folds reduced to two ; they emerge 

 through large spiracles, separated 

 by the interambulacral plates. 

 As for the homologies of the 

 thecal plates, those who believe 

 that there is only one in each 

 interambulacrum regard that one 



as the A, which in the posterior 

 interradius is split in two by an 

 intercalated anal plate. This 

 makes Nucleocrinus a highly 



FIG. XI. 



Section of ambulacrum of Nucleocrinus Ver- 

 neuili, x 10 diam. b.r, brachiole ; L, lancet- 



m->prifll form intrt whirli i<? P Iate ! - s -P, outer side -plate ; s.p, side -plate ; 



lm > 1 ' s, supposed suture between lateral and central 



suddenly introduced an element interambulacrals. 



found in. no other Eublastoid. If, 



however, we accept the view that each interambulacrum has essentially 

 the same composition, viz. three plates, we are able to institute compari- 

 sons with Protoblastoidea. These suggest that the true homologues of the 

 A in Nucleocrinus are the proximal portions only of the plates called deltoid 

 by Etheridge & Carpenter. This latter explanation of the structure of 

 Nucleocrinus permits us to regard it as primitive in all except the hydro- 

 spires, and consists better with its geological age. Schizoblastus, Eth. & Carp. 

 (1882-86), Carboniferous, Britain and N. America, may be described 

 as a Nucleocrinus in which there is only one plate in each of the inter- 

 ambulacra ; this therefore may be called a deltoid, but may well represent 

 the three plates some suppose to exist in Nucleocrinus, since it preserves 

 their peculiar sculpturing. In some species it is of much less relative 

 size. The hydrospires, as in Nucleocrinus, are of simple structure, with 

 one to four folds. In two American species the posterior spiracles 

 are separate from the anus, as in Nucleocrinus; in others they are con- 

 fluent. The plates roofing the mouth, though not quite so stout as in 

 Nucleocrinus, are usually well preserved. Cryptoblastus, Eth. & Carp. 

 (1886), Carboniferous, N. America, is like a Schizoblastus with email 

 A. The hydrospires differ from those of Nucleocrinus and Schizoblastus 

 only in the development of hydrospire- plates (cf. Troostocrinus) which 

 extend right up the sides of the lancet-plate, separating it from the 

 folds and from the walls of the radial sinus. But where the A are 



