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: Asthenosoma and Phormosoma. 
Family 3. ECHINIDA. 
Echinide, Bell, P. Z. S. 1881, p. 417. 
The Echinide are regular ectobranchiate Euechinoidea, in which 
the organs of Stewart are vestigial : they are provided with spheridia. 
The calycinal area is never very extensive, and is often much con- 
centrated; the ambulacral plates always formed of at least three 
plates, so that there 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 ECHININA. 
Echinine, Bell, P. Z. 8. 1881, p. 418. 
Echinide with circular body. 
Three genera: Echinus, Strongylocentrotus, and Sphwrechinus. 
Suborder 2. ITIRREGULARIA. 
Trregularia, Latreille, Fam. Nat. (1825) p. 533; Gray, Cat. Echin. B. 
M. i. (1855) p. 1. 
Kchinides paranormaux ou irreguliers, Gras, Ours. foss. de U’Isére, 
(1848) p. 39; Desor, Ech. foss, (1858) p. xxv. 
lrregulares, Zittel, Hdbuch. d. Paliiont. (1879) p. 510. 
Exocyclica, Wright, Brit. Foss. Echin. Oolitic, 1. (1857) p. 17. 
Euechinoidea in which the anus is in the posterior interradium 
beyond the calycinal aveas. Lantern of Aristotle and auricles 
reduced or lost. 
Grade A. GNATHOSTOMATA. 
Les Clypéastres, 4g. Mém. Soc. Neuchétel, i. (1836) p. 185. 
Clypeastrides, Lovén, K. Svenska Vet.-Ak. Hadlg. xi. no. 7. (1874) p. 32. 
Clypeastroida, Duncan, J. Linn. Soc. xxiii. (1889) p. 142. 
Kchinodermes exocycliques gnathostomes, de Loriol, Echinol. helvét. in 
Pal. Suisse (Pictet), 1875, p. 172. 
Irregular Euechinoidea in which the central mouth is provided 
with teeth and jaws. Ambulacra simple or petaloid, all similar. 
