296 



PHYLUM ECHINODERMATA. 



FIG. 202. Platycrinus trigin- 

 tidactylus (restored after de 

 Koninck from Zittel). 



Fam. 1. Platycrinidae. Calyx formed of a monocyclic base and a 

 circle of 5 large radials. Interradials restricted almost exclusively to the 

 calyx-cover, which consists of firmly connected, usually thick plates and 

 is usually much arched (Fig. 202). No anal plate. Arm-branches 10, 20 or 

 more, generally free from the distichals upwards. There is one primibrach 



(Br. 1) attached to the radial ; the following 

 brachials are free and the arm branches close to 

 its base. Pinnules well developed. The cover- 

 ing plates of the ambulacra are often exposed on 

 the surface of the calyx-cover. Silurian to Car- 

 boniferous. Platycrinus Mill. (Fig. 202), Carboni- 

 ferous, Marsipocrinus Bather. Culicocrinus J. 

 Miill., Cordylocrinus Ang. 



Fam. 2. Hexacrinidae. Monocyclic, 2 or 3 

 basals, first anal plate resting on basals and 

 similar in form to radials. Other plates as in 

 Platycrinidae. Devonian and Carboniferous. 

 Dichocrinus Miinst. ; Arthracantha Williams 

 (Hystricrinus Hinde), calyx plates beset with 

 mobile spines. Hexacrinus Austin, Talarocrinus 

 W. and Sp. 



Fam. 3. Actinocrinidae (Fig. 193). Mono- 

 cyclic. Calyx composed of 3 basals, 5 radials, 



5x2 primibrachs, and 5 X a variable number of secundibrachs. First 

 anal interradial plate resting on basals, the other first interradials upon 

 the radials. Arms 5 to 30 or more, uni- or bi-serial with long pinnules. 

 Calyx-cover usually much arched and ambulacra of disc hidden by inter- 

 ambulacral plates. With or without anal tube. U. Cambrian to Carboni- 

 ferous. Carpocrinus Miill. Actinocrinus Mill. Cactocrinus W. and Sp. 

 (Fig. 193). Strotocrinus M. and W. Batocrinus Casseday. Dorycrinus 

 Roem. Desmidocrinus Ang. Agaricocrinus Troost. 



Fam. 4. Barrandeocrinidae. Monocyclic, 3 basals. Arms biserial, 

 laterally fused with each other, and bent back so as to lie with their dorsal 

 sides against the calyx. Silurian. Barrandeocrinus Ang. 



Fam. 5. Reteocrinidae. Monocyclic or dicyclic. Infrabasals when 

 present 5, basals 4 or 5. Radials separated by a large special anal which 

 supports a vertical row of anals. The spaces on either side of this row, 

 as well as the other four interradii paved with minute pieces. U. Cam- 

 brian. Reteocrinus Bill. Xenocrinus Miller. 



Fam. 6. Thysanocrinidae. Dicyclic, radials in contact laterally ex- 

 cept at the posterior side, where they are separated by an anal plate, 

 U. Cambrian, Silurian. Dimerocrinus Phill. (Gly piaster, Thysanocrinus, 

 Eucrinus). Cyphiocrinus etc. 



Rhodocrinidae. Dicyclic. Calyx composed of 5 infrabasals, 

 X 2 primibrachs and 10 X 1-3 secundibrachs ; interradials 

 The first interradials inserted between the radials and touching 

 The anal interradius is hardly distinguished from the others. 

 U. Cambrian to Carbon. Lyriocrinus Hall, Rhipidocrinus Beyrich, 

 Rhodocrinus Miller. 



Fam. 8. Melocrinidae. Monocyclic, 3-5 basals, radials in contact all 

 round, neither anal nor interradials touching the basals, 2x5 primi- 

 brachs. and 2 to 3 X 10 secundibrachs. Arms 5x2 with secondary 

 branches which bear the pinnules. U. Cambrian to Devonian. Melo- 



Fam. 7. 

 5 basals, 5 

 numerous, 

 the basals. 



