352 CRETACEOUS ECHINOIDEA. 



Family v.— ECHINOCONID^, Wright {^^. 198). 



Test thin, circular, elongated or pentangular, elevated or depressed. Ambulacral 

 areas narrow. Interambulacral areas wide. Both areas covered with numerous small 

 perforated and crenulated tubercles. Poriferous zones simple, narrow, straight. Pores 

 unigeminal except near the peristome, where they are trigeminal. Oral opening on 

 under side central, circular, or pentagonal, notched into ten nearly equal lobes armed with 

 five jaws. Apical disc central on upper side, composed of five ovarial and five ocular 

 plates, madriporiform body very large, extending from the right antero-lateral ovarial 

 plate into the centre of the disc. Anal opening variable in position on upper, lower, or 

 marginal surface. Spines small, short, subulate. Range of family, from the Lower Oolites 

 to Upper Chalk. Three British Cretaceous genera : Discoidea, Echinoconus and 

 HoLECTYPUs, with eleven species (pp. 198 — 234). 



Genus 12. — Discoidea, Klein (p. 199). 



Test thin, circular, hemispherical, elevated. Ambulacral areas straight, narrow. 

 Interambulacral wide, covered with numerous small, perforated, crenulated tubercles. 

 On the inner side surface of the interambulacral plates near the poriferous zones are 

 thick, shelly processes projecting inwards and forming small septa. Poriferous zones 

 very narrow. Oral opening small, circular. Peristome decagonal, marked by slight 

 notches. Anal opening oval, always inferior, and placed between the peristome and 

 border, covered with small irregular granular plates. Apical disc solid, forming a slight 

 projection on the summit of the test, having four or five perforated and one imperforate 

 ovarial plates. Spines short and stout. Range of genus, from the Upper Greensand to 

 the White Chalk. Five British Cretaceous species (pp. 200—213). 



Genu8 13. — Echinoconus, Breynius (p. 213). 



Test thin, round, oval or pentagonal, enlarged a little before and slightly contracted 

 behind. Upper surface more or less elevated, rounded or conoidal, under surface flat, 

 sometimes concave. Ambulacral areas lanceolate. Interambulacra Avide. Tubercles 

 perforated and crenulated. Poriferous zones straight, narrow. Pores unigeminal 

 becoming trigeminal at the peristome. Oral opening central, small, circular. Peristome 

 subdecagonal, notched. Jaws present. Apical disc solid, placed at summit, having 

 four perforate and one imperforate ovarial plates. Ocular plates very small and interposed 

 between the angles of the ovarials. Spines small, acicular. Range of genus, from the 

 Upper Greensand to the Upper Chalk. Five British Cretaceous species (pp. 213—231). 



