IV. Trenton Limestone Edrioasters. 1914 170 



Family EDRIOASTERIDJE. 



Edrioasteroidea with a theca composed of separate and relatively 

 thin plates, and apparently without permanent attachment ; with 

 a subvective skeleton partly visible in apical aspect, and composed of 

 floor-plates and movable cover-plates, having pores between the 

 floor-plates ; interradials of oral face continuous with those of apical 

 face ; central area of apical face covered with more flexible integument, 

 bearing smaller plates ; a hydropore (probably always) pierces an 

 adoral interradial of the posterior interradius. 



The genera are Edrioaster E. Billings, 1858; Aesiocystis Miller 

 andGurley, 1894 ; Dinocystis Bather, 1898 ; Lehetodiscus Bather, 1908. 



Aesiocystis comprises a single species, A. priscus, based on four 

 imperfect specimens from the Trenton Group of Mercer Co., Ky. (Bull. 

 Illinois State Mus. Nat. Hist., No. 5, p. 13, pi. ii). The genus was 

 placed by its authors in the family Hemicystidae, but their description 

 and figures show that it was almost certainly an Edrioasterid ; indeed, 

 my chief doubt is whether the species was not actually an Edrioaster. 



Edrioaster. 



An Edrioasterid with pores of subvective groove within the tract 

 protected by cover-plates ; with interradials all tessellate and 

 separated from the central apical region by a frame of stouter 

 plates ; on the apical face the peripheral plates are variable in size 

 but not minute, the central plates are minute and tend to imbricate. 



Edrioaster bigsbyi E. Billings. 



An Edrioaster with height of theca about one-third of diameter, 

 periphery roughly circular ; with rays clearly raised above general 

 surface, and floor-plates clearly demarcated from interradials ; rays 

 I, II, III, IV have a contrasolar curve, ray V a solar curve, ray IV 

 bends upwards within r. ant. interradius; interradials of oral face 

 bear coarse, sparse pustules. 



This is the genotype. 



Edrioaster buchtanus E. Forbes sp. 

 An Edrioaster with height of theca one-half the diameter or more, 

 periphery sub-pentagonal with convex interradii ; with rays apparently 

 not raised above general surface, but with floor-plates clearly demar- 

 cated from interradials ; rays all have a solar curve ; interradials 

 slightly pustulate ? 



Edrioaster saratogensis Ruedemann. 



An Edrioaster with depressed theca (height probably about one- 

 fourth diameter), periphery sub-circular to sub-pentagonal with 

 convex interradii ; with rays clearly raised above general surface, 

 and floor-plates not regularly demarcated from interradials ; rays all 

 have a solar curve ; interradials relatively few, and all, as well as 

 cover-plates, densely and finely granulose. 



This diagnosis is based partly on the description and figures of 

 Dr. Ruedemann (1912, Bull. KY. State Mus., No. 162, p. 86, pi. iii), 

 partly on squeezes which he was so kind as to send me. Dr. Ruedemann , 

 apparently being acquainted with none of my previously published 



