CYSTIDEA. 309 



correct or not, it appears certain that they could not have been tubes 

 leading from the exterior into the body-cavity of the animal, because they 

 are in so many cases closed by the layers of epi- and hypostereom above 

 referred to. Structures very similar to pore-rhombs, having the form of 

 deep folds crossing the sutures between the plates are found in many fossil 

 Crinoids (see especially Porocrinus, Carabocrinus and Hybocrinus). The 

 ordinary structure of the mesostereom, simulating folds, is best seen in 

 genera with large plates such as Marsupites and Crotalocrinus. 



In no Cystid except the Edrioasterida are pores present along the ambu- 

 lacra! grooves which can have served for the passage of tube -feet. Whether 

 tube-feet were present is, however, another question. 



The Cystids make their appearance in the Cambrian in which and in the 

 Silurian they are represented by a great diversity of forms. They die 

 out in the Permian. 



Order 1. AMPHOBIDA. 



Forms without radial symmetry. Often with irregular arrangement of 

 thecal plates ; usually with a stalk. Body often bilaterally compressed 

 with two food-grooves (Trochocystis}. The plates may possess canals, 

 but their ends are not open. A variable number of arm-like processes 

 may be present in some genera. Aristocystis Bar. (Fig. 214), upper 

 Cambrian. Dendrocystis Bar. (Fig. 212),U. Cambrian. 



Order 2. RHOMBIFERA. 



Usually with a stalk. With radial symmetry of the ambulacral 

 grooves and in some forms of the thecal plates. The ambulacral 

 grooves usually extend on to processes of the theca (brachioles, arms) 

 arising at the edge of the mouth (exo thecal) (Fig. 216) or they may 

 extend outwards for a certain distance on the theca, but in this case also 

 they are exothecal as they do not lie on the thecal plates, but on special 

 plates. With pore-rhombs. When the plates are large and regular they 

 are arranged in cycles and the base is dicyclic, i.e. infrabasals, basals, and 

 radials are present. In Caryocrinus and Hemicosmites there are 6 basals 

 and C radials, in most others 5. Echinosphaera Wahl. (Fig. 216), arms 

 unknown, U. Cambrian. Arachnocystis Neum., usually three arms, U. 

 Cambrian. Cystoblastus Volb. (Fig. 213), U. Cambrian, may possibly have 

 possessed pinnules arising from the edges of the ambulacral grooves, with 

 4 infrabasals, 5 basals and 5 radials. Pleurocystis Bill, U. Cambrian, with 

 two arms. Callocystites Hall (Fig. 219), Silurian, 

 with 4 pectinated rhombs, with ambulacral 

 grooves on the calyx, some of which bifurcate. 

 Echinoencrinus H. v. Mayer, 3 small arms, U. 

 Cambrian. Lepadocrinus Conrad, Silurian. Caryo- 

 crinus Say, arms 6-13 in number, Silurian (Fig. 

 218). Hemicosmites v. Buch, U. Cambrian. 



Order 3. DIPLOPOBIDA. 



With radial symmetry ; with ambulacral grooves 

 on the theca (epithecal, Fig. 221), provided with 

 lateral pinnules (Fig. 215), and continued ontoter- lin, from Delage and 



minal brachioles (Fig. 220) ; with diplopores, with- Herouard). 4nanus. 



out rhombs. Eucystis Ang. (Fig. 220), U. Cam- 

 brian. Protocrinus Eich. (Fig. 217), U. Cambrian. Mesocystis Bather 

 (Mesites Hoff.), U. Cambrian, with 5 ambulacra with covering plates 



