310 THE RCHINOIDEA 



Polycyphn*, Ag. ; Pedinothnria, Greg. ; Micropcdina-, Cott. ; Codechinus, 

 Des. ; Echinopediiia, Cott. ; Echinoihrix, Pet. ; Astropyga, Gray. 



FAMILY 5. CYPHOSOMATIDAE. Diademina in which the ambulacral 

 plates are compound ; they are high with from three to seven pore-pairs 

 in an arc ; the ndoral and supra-adoral, and sometimes also the aboral 

 plates are primaries. The remaining constituents 

 are demi-plates (Fig. XXVIIL). Genera Actino- 

 phyma, Cott. & Gauth. ; Cyphosonw, Ag. ; Leiosoma, 

 Cott. & Triger ; Gauthieria, Lamb. ; Th.ylechinus t 

 Pomel ; Coptosoma, Des. ; Micropsis, Cott. ; Orthe- 

 Fio xxvin chinus, Gauthier (syn. Gagaria, Dune.) ; Triplacidia, 



Ambulacral plate of Bit tner. 

 Cypkosoma, composed of In the normal compound ambulacral plates of 



three primaries and two ,v TV i .L-J i i i L a A.I 



demi-plates. the Diadematidae each plate consists of three prim- 



aries ; but, as in the case of Fig. XXIV., an extra 



demi-plate sometimes appears below the uppermost primary. This is the 

 link between the typical Diadematidae and Cyphosomatidae. Demi -plates 

 appear in Diadematids in the Middle Oolites ; the Cyphosomatidae begin 

 in the Upper Oolites and attain their maximum in the Cretaceous. The 

 last members of the family lived in the Eocene. 



FAMILY 6. ECHINOTHURIDAK. Diademina in which the test is more 

 or less flexible ; the plates are thin and usually separated by membrane. 

 Apical system rudimentary (Fig. X.) ; ambulacral plates triserial, arranged 

 typically in triplets of a central primary between two klasma-plates. In 

 >ne genus three triplets unite together to form a single plate. This 

 interesting family was founded by Wyville Thomson to include the 

 Chalk fossil Echiiiothiiria, S. P. Woodw., and some living Echinoids 

 dredged by the Porcupine Expedition. As the tests are flexible and 

 the plates overlap, the family was at first compared with the Palaeozoic 

 Echinoids. P. and F. Sarasin argued from the rudimentary nature of the 

 apical disc, and from the great size of the " Stewart's organs " and the 

 presence of powerful radial muscles (the two latte A characters being very 

 noticeable in a new species of Asthenosoma described by those authors) that 

 the Echinothuridae were a primitive family of Echinoids, and helped to 

 establish the origin of this class from a Holothuroid ancestor. Neviani 

 accepted this conclusion. But, as has recently been shown (Gregory, 

 35), tue family is an offshoot from the Pedinidae ; the genus Pcdiiw- 

 thuria helps to bridge the gap between Pedina and the oldest known 

 Echinothurid Pelaneckinus. 



SUB-FAMILY 1. PELANECHININAE. Echinothurids of which the am- 

 bulacral plates are compound ; those near the apex consist of two demi- 

 plates and a large middle primary. Those at the ambitus consist of 

 three sets of three plates united into a single polyporous plate ; each 

 triplet of this compound plate consists of a primary between two demi- 

 plates or klasma- plates. Genus Pelanechinus, Keeping; Corallian, 

 Wiltshire. An admirable account of the genus has been given by Groom 

 (37). SUB-FAMILY 2. ECHINOTHURINAE. Echinothuridae in which the 

 ambulacral plates are simple and free ; they consist of triplets, each com- 

 posed of a large central primary, and with an isolated klasma-plate above 



