THE EDRIOASTEROIDEA 



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connect this family with the more primitive forms of Agelacrinidae. 

 Edrioaster, Billings (1858), Ordovician, Canada (Fig. VI.), also includes 

 " Agelacrinite* Buchianus" Forbes (1848), from Wales. Theca about half 

 as high as wide, depressed slightly at oral pole. Kays curved, all or 

 some sinistral or dextral, passing on to under surface of theca ; arnbu- 

 lacral groove floored by alternating plates [? adambulacrals], between which 

 were pores [? for podia] ; with covering -plates, i.e. ambulacrals, over 

 the groove. Interambulacrals non- imbricate. Under- surface of theca 

 excavate, its central region composed of a flexible membrane set with 

 minute imbricating plates, and in a frame of about 11 large plates. 

 Whether the animal was attached by the central part of this membrane 

 is doubtful ; immediately round the centre this is evaginated in five lobes, 

 apparently caused by the pressure of some internal organs [?gonads], 



amb - 



amb 



ad 



FIG. VI. 



Edrioaster Bigsbyi. 1, oral surface, with covering-plates (amb) on the anterior and left 

 anterior grooves, but removed from the others, which show only the side- or flooring-plates 

 (ad), between which are pores (p). The greater part of the subpentagonal peristome (ps) is 

 roofed by enlarged covering-plates, ia, interambulacrals, one of which is a madreporite (Af), 

 2, section across the same specimen through the right anterior radius and left posterior 

 interradius. The covering-plates are removed except just over the peristome, and in the am- 

 bulacrum seen in section on the left. /, frame of stouter plates ; m, membrane with imbricating 

 plates, thrown into five lobes (I). 3, section across an ambulacrum, with plates in situ 

 covering ventral groove (w/). Dotted surfaces are the natural edges of the plates, ruled surfaces 

 are cut through the plates. (All slightly diagrammatised from a specimen belonging to the 

 Canadian Geological Survey.) 1 and 2 are nat. size. 



which must have acquired pentamerous symmetry. Dinocystis, Bather 

 (1898), Uppermost Devonian, Belgium, has a slighter frame on the aboral 

 surface, and the surrounding region composed of a thin flexible integument 

 containing narrow imbricating ossicles ; otherwise like Edrioaster. 



FAMILY 4. STEGANOBLASTIDAE. Edrioasteroidea, with a rigid theca 

 composed of plates relatively larger and thicker than in other families of 

 this class ; these include elements comparable to the RR and BB of 

 Blastoidea ; BB attached to a stem, probably short ; ambulacra descend 

 into the radials. Genus Steyanoblastus, Whiteaves (1897, originally 

 described as Astrocystites, name preoccupied), Ordovician, Canada (I .g. 

 VII.). The remarkable resemblance to Asteroblastus y insisted on by its 

 founder, suggested the reference of Steyanoblastus to the Protoblastoidea 

 (Bather, 1899); but tlie ambulacra are now known to have essentially 

 the same structure as in Edrioaster, while the absence of brachioles may 

 be maintained with confidence. Theca piriform, its plates strongly marked 



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