THE EDRIOASTEROIDEA 



207 



FAMILY 1. AGELACRINIDAE. Edrioasteroidea with a theca composed 

 mostly of thin plates, flexible, attached temporarily or permanently by 

 the greater part of the aboral surface ; with ambulacra confined to the oral 

 surface. Genera Stromatocystis, Pompeckj (1896), Cambrian, Bohemia, 

 seems to have had a somewhat flexible theca of non-imbricate plates 

 (Fig. I.). Rays straight and extending to the margin, imposing on the 

 theca a subpentagonal outline ; composed of stout and long alternating 

 side-plates, along the outer margins of which are pores [for podia 1] 

 while along their inner margins is a groove, protected by minute 

 covering-plates. Four large and many small plates [modified side- and 

 covering-plates] surround and cover the mouth. Interambulacrals hex- 

 agonal, united by stroma- strands. Under side of theca composed of 

 irregular plates, without evidence of stroma-strands, and larger towards 

 the middle. The animal was probably sessile on its under surface, but 

 perhaps not fixed permanently. Cystaster, Hall (1872, Thecocystis, Jak.), 

 Ordovician, Ohio, is also primitive (Fig. II.). Theca sac-like, composed of 



Fi<;. III. 



Aijelacrlnus hamiltonensis, oral surface, 

 wwft, marginals, consisting of the large 

 internal (i) and the small external (c) ; M, 

 supposed madreporite. Other letters as 

 before. (After Hall.) x 2. 



Fio. IV. 



Lcpidodiscus cincinnaten.-'is, oral surface. 

 Lettering as before. (After Hall.) x \ . 



minute plates, not always attached (?) ; rays straight, composed of alternat- 

 ing covering -plates supported on side -plates ; no hydropore observed. 

 Hemicystis, Hall (1852), Ordovician and Silurian, N. America and 

 Bohemia, shows an advance on Cystaster in the imbrication of the 

 thecal plates, and their differentiation into larger interambulacrals, 

 and a zone of smaller marginals. This leads on to Agelacrinus 

 and its allies Streptaster and Lepidodiscus, all which are characterised 

 by the curvature of the rays, sinistrally, dextrally, or both, by the 

 elaboration of the marginal zone, and by their flattened sessile habit, 

 being usually attached to brachiopod shells. The type -species of 

 Agelacrinus is the Devonian A. hamiltonensis, Vanuxem (1842), (Fig. 

 III.), in which the anterior and two left-hand rays curve sinistrally, 

 the two others dextrally ; the interambulacrals are large, non-imbricate, 

 and radiately ridged ; there is a border of large plates, with an outer 



