208 THE EDRIOASTEROIDEA 



border of small plates. Lepidodiscus, Meek & Worthen (1868), Ordovician 

 to Carboniferous, has imbricating interambulacrals and a border of small 

 imbricating plates ; plates forming the floor of the ambulacra, and a pen- 

 tagonal internal frame around the mouth, have been described by Miller & 

 Faber (1892), but more details are needed; Hall figures right posterior 

 ray as dextral, and the rest as sinistral (Fig. IV.) ; other arrangements 

 may obtain, but the rays adjacent to the anus always curve towards it. 

 The Devonian Haplocystis, C. F. Roemer (1855), is known from an internal 

 cast, apparently proving that the ambulacra were floored by a single 

 row of plates [? fused adambulacrals], between the edges of which were 

 pores [for passage of podia]. The Ordovician Streptaster, Hall (1872), 

 has all its rays sinistral. Discocystis, Gregory (1896), based on Echino- 

 discus optatus, Worthen & Miller, is doubtful. 



FAMILY 2. CYATHOCYSTIDAE. Edrioasteroidea with the theca composed 

 on the oral surface of five deltoids surrounded by marginals, but below 

 of a fused solid mass of stereom, with irregular longitudinal sutures ; per- 

 manently attached by the aboral surface as by an encrusting root ; 

 ambulacra confined to oral surface. Genus Cyathocystis, Schmidt (1880), 

 Ordovician, Esthonia (Fig. V.), is not far removed from Stromatocystis and 

 Cystasterj but the trend of its evolution is quite away from that of either 

 Edrioaster or Ayelacrinus. Upper ambulacral surface of theca bordered 

 by a pentagonal frame of 40 marginals ; rays straight, with a single 

 series of covering-plates ; five larger plates cover the mouth. The 

 minute plates that formed the theca of Cystaster are here fused into solid 

 masses ; thus there arise between the ambulacra 5 large A, and below the 

 marginals a massive cup, fixed to some foreign body by its base, and 

 occasionally marked by obscure longitudinal sutures, irregular in number 

 and position, but never more than 5. Anus, with pyramid of 5 plates, 

 lies between a A and the adjacent marginals ; no hydropore observed. 



c.p. 



Cyathncystis Plaittinae. 1, two 

 individuals growing on a pebble of 

 rolled coral, and seen from the side. 

 (After Schmidt.) Nat. size. 2, 

 oral surfacp. As, anus ; A, inter- 

 ambulacrals fused into deltoids ; 

 in-ill, marginals ; c.p,covering-plates, 

 partly removed from the posterior 

 rays exposing fp, which are not a 

 distinct series of flooring -plates, 

 as represented, but ledges project- 

 ing from the under side of the 

 deltoids. (Diagrammatised from 

 Schmidt.) x 3 diam. 



FAMILY 3. EDRIOASTERIDAE. Edrioasteroidea with flexible theca com- 

 posed of thin plates ; attached, if at all, by a small central portion of the 

 excavate aboral surface ; ambulacra pass on to aboral surface. Genera 

 Aesiocystis, Miller & Gurley (1894), Ordovician, Kentucky, has a sub- 

 pentagonal theca, with height two-thirds the width ; rays wide and straight, 

 with large covering-plates ; interambulacrals non-imbricate. Podial pores, 

 madreporite, and abactinal surface unknown ; but the genus appears to 



