APPENDIX I. 



SUMMAEY^ OE THE EEITISH CRETACEOUS EOHINOIDEA. 



Order— ECHINOIDEA, Wrigld (p. 19). 



Body-shell {test) spheroidal, oval, cordate, or depressed, without arras, furnished with 

 a distinct mouth {oral opening), whose border {peristome) is sometimes simple, some- 

 times lobed, always placed either in the centre or forwards on the under side ; when the 

 oral opening is not central its position marks the anterior region ; armed with five calca- 

 reous sets of plates {jaws), or not armed {edentulous). Anal opening {vent, peri- 

 procte, discal opening) variously situated on the upper {dorsal) or under {basal) side in 

 the centre {central) or away from the centre {excentral), or in intermediate positions on 

 the marginal border {circumference, ambitus, equator). Body enclosed in a shell {test) 

 composed usually of twenty, sometimes of more than twenty (as in the family of the 

 Palgeozoic PerischoecJunidce), columns of calcareous plates, forming in either case ten 

 areas ; plates either solidly connected or capable of movement. Eive of the areas {ambu- 

 lacral) narrow or wide, containing each two rows of apertures {poriferous zones) for the 

 passage (in the living state) of retractile suckers {ambulacral tubes). The other five areas 

 {inter ambulacral) more or less wide, destitute of sucker pores. Ambulacral pores dis- 

 posed in single pairs [unigeminal), double {bigeminal), or triple oblique {trigeminal). 

 Ambulacral pore-columns {areas) sometimes continuous from the peristome to the summit 

 {complete), sometimes confined to the upper surface of the test {interrupted), or forming 

 re-entering curves {petaloid). Surface of test studded with tubercles {primary, secondary, 

 and miliary), possessing spines of various forms and dimensions, solid or hollow, smooth, 

 striated, serrated. Spines articulated on the rounded upper part of a tubercle {mamelon) 

 which rises from a conical process {boss). Base of tubercle surrounded by a round, oval, 

 smooth, excavated space {areola or scrobicule). Summit of test marked by an apical 

 {genital) disc, composed generally of five genital and five ocular plates, usually in contact 

 and central. Cutaneous surface of shell, especially near the mouth, bearing in the living 

 stage small, tripartite, pincer-like bodies {pedicellarice), placed on a short stalk, whose 



^ Compiled by the Rev. Prof. Thos. Wiltshire, M.A„ F.G.S., Hon. Sec. Pal. Soc. 



