30 ECHINODERMA. 



or may not have spheridia. The calycinal area is extensive, 

 but its constituents are much reduced in size ; the ambulacral 

 plates consist of a primary plate only, or have half-plates united 

 with them, or of isolated half-plates, and they imbricate and are 

 continued on in several rows on to the peristomial membrane ; the 

 interambulacral plates also imbricate. Perignathic arches radial 

 and complete. 



Two genera : AstJienosoma and Phormosoma. 



Family 3. ECHINID.E. 



Echinidee, Bell, P. Z. 8. 1881, p. 417. 



The Echinidae are regular ectobranchiate Euechinoidea, in which 

 the organs of Stewart are vestigial : tbey are provided with spheridia. 

 The calycinal area is never verjr extensive, and is often much con- 

 centrated ; the ambulacral plates always formed of at least three 

 plates, so that tbere are three pairs of pores at least in each, and 

 only one pair from each radius is found on the peristomial mem- 

 brane. Perignathic arches radial and complete. Podia homoio- 

 podous. Spines acanthodictyote, acanthosphenote, polycyclic ; C- 

 shaped spicules always among the calcareous deposits. 



Subfamily ECHININ^E. 

 Echmhije, Bell, P. Z. S. 1881, p. 418. 



Echinidae with circular body. 



Three genera : Echinus, Strongylocentrotus, and Splicer echinus. 



Suborder 2. I R R E G U L A R I A. 



Irregularia, Latreille, Fam. Nat. (1825) p. 533 ; Gray, Cat. Echin. B. 

 ^ M. i. (1855) p. 1. 

 Echinides paranormaux ou irreguliers, Gras, Ours. foss. de VIsere, 



(1848) p. 39; Besor, Ech.foss. (1858) p. xxv. 

 Irregulares, Zittel, Hdbuch. d. Palaont. (1879) p. 510. 

 Exocyclica, Wright, Brit. loss. Echin. Oolitic, i. (1857) p. 17. 



Euechinoidea in which the anus is in the posterior interradium 

 beyond the calycinal ai'eas. Lantern of Aristotle and auricles 

 reduced or lost. 



Grade A. GNATHOSTOMATA. 



Les Clypeastres, Ag. Mem. Soc. Neuchdtel, i. (1836) p. 185. 

 Clypeastrides, Loven, K. Svenska Vet.-Ak. Hdlg. xi. no. 7. (1874) p. 32. 

 Clypeastroida, Duncan, J. Linn. Soc. xxiii. (1889) p. 142. 

 Echinodermes exocycliques gnathostomes, de Loriol, Echinol. helvet. in 

 Pal. Suisse (Pictet), 1873, p. 172. 



Irregular Euechinoidea in which the central mouth is provided 

 with teeth and jaws. Ambulacra simple or petaloid, all similar. 



