132 THE FOSSIL ECHINOIDEA 



Family CLYPEASTEIBM. 



Subfamily EUOLYPEABTBIDA 



Genus ECHINOCYAMUS, mn Phelsum, 1774. 



Test of small size, more or less depressed, oval, oblong. 



Ambulacral summit central ; apical system compact, four generative pores, five 

 ocular pores. 



Ambulacra indistinct and scarcely petaloid, open at their extremity. Poriferous 

 zones with few pores, arranged in widely spaced pairs. Pores of a pair round and not 

 united by a conjugating furrow. 



Interambulacral areas of the test supported internally by a pair of simple vertical 

 partitions radiating from the margin toward the peristome. 



Peristome central, subpentagonal, with high auricles internally. 



Periproct oval or rounded, placed between the peristome and the posterior margin 

 of the test. 



1. EcHiNOCTAMUS NUMMULiTicus, Duncan & Sladen. Plate XXV, Figs. 14-20. 



Form subdepressed, but rather high for the genus ; marginal contour oval, slightly 

 more contracted in front than behind. Abactinal area convex, almost subconoid, with 

 the greatest height subcentral or slightly excentric in front ; seen in longitudinal profile 

 the posterior slope is somewhat straighter or less arched and less bombous than the 

 anterior slope, and the anterior margin is thicker and more tumid than the posterior. 

 The transverse profile is regularly convex, and its outline approaches the conoid. The 

 actinal surface is flat, the margins of the area rounding gently on to the sides, which 

 are well rounded. There are slight depressions along the median radial lines of the 

 actinal surface, more distinct in some specimens than others, which give a somewhat 

 undulating or faintly tumid character to the area. 



The apical disk is very small, and the four genital pores are scarcely distinguishable 

 from the tentacular pores of the ambulacra. The anterior pair are placed closer 

 together than the posterior. Immediately between the anterior pair of pores is a 

 rather smaller pore, which is probably the madreporic foramen. The ocular pores are 

 exceedingly minute, and only visible under high magnification and careful illumination : 

 that of the odd anterior radius is placed more outward than the madreporic pore, and 

 stands at the apex of a triangle, almost equilateral, whose base would be represented by 

 a line joining the anterior pair of genital pores. The anterior pair of ocular pores 

 stand midway between the antero- and postero-lateral genital pores, but more distant 

 from the centre, and are consequently wider apart than the anterior pair of genital pores. 

 The posterior pair of ocular pores are placed behind the posterior pair of genital pores, 

 that is to say further outward, away from the centre ; they are, however, much nearer 

 together than the posterior genital pores, being about the same distance apart as the 

 anterior pair of genital pores. 



The ambulacral petals are broad, short, subequal, and very rudimentary in character. 



