THE CYSTIDEA 



57 



All plates except the minute ones at the base were united by strong 

 stroma-strands across their sutures, and these form pore-rhombs. 



FAMILY 4. TIARACRINIDAE. Ehombifera in which the plates forming 

 the sides of the theca are arranged in not more than two circlets, and the 

 plates of each circlet are transversely united by strongly marked pore- 

 rhombs. The genera, though clearly separated as offshoots from the 

 Ehombifera, do not form a very coherent group, and need careful description 

 by a well-informed worker. Tiaracrinus, Schultze (1867, Syn. Staurosoma, 

 Barrande, 1887), Devonian, Eifel, and Bohemia. Theca cup-shaped or a 

 truncate spheroid, the sides composed of four large, interradially situated 

 plates, one of which is (? only in some cases) horizontally bisected. These 

 plates are united by strongly marked stereom- folds, raised above the 

 surface, and forming demi-rhombs. The composition of the oral surface is 

 unknown, but there seems to have been a central mouth, with food-grooves 

 radiating from it towards the margin, where brachioles probably arose. 

 An anus seems to have pierced the margin at the summit of one of the 

 triangular side-plates. Stem-facet four-sided, with angles radial ; lumen 

 small. Rhombifera, Barrande (1867 and 1887), Ordovician, Bohemia. 

 Theca elongate, triangular in section ; appears composed of two circlets a 

 lower, of three plates united by strong stereom-folds, visible exteriorly only 

 as terminal pores outlining " pore-rhombs " ; an upper, of six (?) plates, of 

 which three pairs are united by pore-rhombs, similar to those of the 

 lower circlet, and vertically above, not alternating with them. Oral 

 region unknown. Aboral region passes gradually, by smaller plates, into 

 a cylindrical stem. The structure of the pore-rhombs and the trimerous 

 symmetry suggest comparison with Caryocrinidae. Rhombifera mira, 

 Barr., is usually considered to be a 

 Stephanocrinus. Aethocystis, S. A. Miller 

 (1892), Silurian, Indiana, may be placed 

 here provisionally. 



FAMILY 5. MALOCYSTIDAE. Rhombi- 

 fera in which thecal plates are numerous 

 and indefinitely arranged. Radial folds 

 of stereom often pronounced, but minor 

 rhomb-like striae not clearly seen. Food- 

 grooves on exothecal processes passing 

 over the theca and bearing brachioles. 

 Columnals (when known) in a single 

 series. This Ordovician family shows 

 the independent evolution of a structure 

 common in a later family, Glyptocystidae, 

 viz. the extension from the mouth over 

 the theca of series of alternating plates, 

 supporting a single series of brachioles. 



The alternating series is not SO com- some brachioles ; Br 1 , facet for attach- 

 T , i -/-IT i-i jii_ mentofsame. Original, from specimen 



plicated as in Glyptocystidae, and the belonging to Dr. G. J. Hinde. 



food-groove passes, not on the top of it, 



but along its sides. The main grooves appear in all cases to be reduced 



to two ; but these may branch and wind round the theca as in the later 



FIG. XIX. 



Amygddlocystis florealis. 



1, from side ; 



