CLASS I CYSTOIDEA 159 



majority of Cystids, extending from the Cambrian to the Lower Carboniferous. 

 Two geriera, Agelacriniis and Edrioaster, are fairly abundant in certain localities 

 of North America, but the others are rare. 



Family 1. Agelacrinidae Hall. 



Calyx composed mostly of thin plates, flexible, attached temporarily or per- 

 manently hy the greater pari of the aboral surface ; ambulacra confined to the oral 

 surface. Cambrian to Carboniferous. 



Agelacrinus Vanuxem (Fig. 250). Calyx in the form of a depressed or 

 convex disk, stemless, and attached by the entire under surface ; composed of 

 numerous, small, polygonal, usually imbricating 

 plates, which are perforated by fine, usually 

 conjugate pores. Mouth surrounded by four 

 oral plates ; radiating from this are five small, 

 more or less curved food-grooves, which are 

 embedded in the disk, and are protected by a 

 double row of covering plates. Ordovician ; 

 North America, Rhineland and Bohemia. Rare 

 in Silurian and Devonian. 



Stromatocystis Pompeck j. Cambrian; Bohemia. 



Cystaster Hall {Thecocystis Jaekel). Streptaster fig. 250. 



Hall. Ordovician ; Ohio. 'Agelacrinus cindnnatiensis Roemer. 



. Hemky Stiles Hall. Ordovician and Silurian ; SÄ'Äe SÄlieÄren'^tJ; 



North America and Bohemia. ^^^t of liafinesqulna altemata (Conrad). 



Haplocystis Roemer. Devonian ; Rhineland. 



Lepidodiscus Meek and Worthen. Devonian to Carboniferous ; North 

 America. Discocystis Gregory. Carboniferous ; North America. 



Family 2. Oyathocystidae Bather. 



Calyx composed on the oral surface of five deltoids surrounded by margmals, hut 

 below of a fused solid mass of stereom, with irregulär longitudinal sutures ; ambu- 

 lacra confined to oral surface ; permanently attached by the aboral surface, as by an 

 encrusting roof. Ordovician. 



Cyathocystis Schmidt. Ordovician ; Esthonia. 



Family 3. Edrioasteridae Bather. 



Calyx flexible, composed of thin plates ; attached, if at all, by a small central 

 portion of the aboral surface ; ambulacra pass on to the aboi'al surface. Ordovician 

 to Devonian. 



Edrioaster Billings (Aesiocystis Miller and Gurley) (Fig. 251). Ordovician ; 

 Canada and Kentucky. 



Dinocystis Bather. Devonian ; Belgium. 



^ Cyclocystoides Billings and Salter. Ordovician ; North America and Great 

 Britain. Probably a Cystid, but not sufficiently known to be assigned to any 

 particular family. 



