THE BLASTOIDEA 



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. Koemer 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- u.r...^& 



ture is shown in Figs. X. 2 and ^=JL>W 



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 Flo XI 



intercalated anal plate. This gection of ambulacrum O ' f y llfleocrinV8 Ver _ 



makes Nudeocrinus a highly neuili, x 10 diam. b.r, brachiole; /,, lancet- 

 SDCciiilisGcl foi'iii into wliicli is k ' *i * i * * i * *"~i *^^ 



suddenly introduced an element iuterainbulacwb. 

 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 Nudeocrinus are the proximal portions only of the plates called deltoid 

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

 Nudeocrinus permits us to regard it as primitive in all except tne hydro- 

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

 (1882-86), Carboniferous, Britain and N. America, i^ay be described 

 as a Nudeocrinus 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 Nudeocrinus, since it preserves 

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

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

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

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

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

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

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

 A. The hydrospires differ from those of Nudeocrinus and Sckizoblastut 

 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 



