160 



ECHINODERMATA— PELMATOZOA 



PHYLUM IV 



Family 4. Steganoblastidae Bather. 



Calyx rigid, composed of plates relatively larger and thicker than in other 

 f amilies of this group, including elemenfs comparable to the radials and basals of 

 Blastoidea. Ämbulacra descend into the radials. Ä short stem present. Ordovician. 



SteganoUastus Whiteaves (originally described as Astrocystites, name pre- 

 occupied). In all the prominent external characters resembling a Blastoid, 

 but careful study of the type specimens by Bather has shown the ämbulacra 

 to have essentially the same structure as in Edrioaster, and that brachioles are 

 absent. Ordovician ; Canada. 



am 



P anit yg gj 



Fio. 251. 



Edrioaster bigsbyi Bill. Ordovician ; Ottawa, Canada. 1, Oral surface with covering plates (amh) on two 

 of the grooves, and side- or flooring-plates {ad) on the others, x 1/1. 2, Vertical section of same, 1/1. 3, Sec- 

 tion across an ambulacrum, enlarged. Ad, flooring-plates ; amb, covering-plates ; as, anus ; ia, interambu- 

 lacrals ; M, madreporite ; m, membrane with imbricating plates, thrown into live lobes (0 ; /, frame of stouter 

 plates ; ps, subtegminal peristome ; 2\ pores ; vg, ventral groove (after Bather). 



The following generic names have been incorrectly applied to Cystids : 



Ascocystües Barrande. Probably a Camerate Crinoid. 



Camarocrinus Hall. {Lobolithus Barr.). Inflated or bulboiis root of the Camerate 

 Crinoid, Scyphocrinus. 



Cardiocystis Barrande. Indeterminable. 



Orinocystis Hall. Probably a Camerate Crinoid. 



Cyclocrinus Eichwald (Pasceolus Billings). Not an Echinoderm, 



Dictyocrinus Conrad. A Receptaculite. 



Hyponome Loven. The ejected disk of a Comatulid. 



Lichenocrinus Hall. The terminal stem-plate or root of some Pelmatozoan. 



Neocystites Barrande. Probably the root of a Pelmatozoan. 



Forocrinus Billings. An Inadunate Crinoid. 



Range and Distribution of the Cystoidea. 



The Cystideans, a wholly extinct class, are the oldest known members of 

 the Pelmatozoa. They are represented in the Cambrian by a number of poorly 

 preserved forms, whose affinities are in many cases doubtful {Protocystites, 

 Macrocy Stella, Eocystites, Lichenoides, Trochocystites). They attain their maximum 

 development in the Ordovician and Silurian, whereupon they suddenly dimiiiish 

 in numbers, and probably disappear in the early Carboniferous. Of the 250 

 species that have been described, scarcely a dozen are found in strata above 

 the Silurian. 



