Studies on the Echinoidea. No. XI. 423 
Transverse diameter greatest at ambulacra II and IV (which form 
blunt angles), thence lessening gradually to areas I and V (which 
are not at all prominent). A slight posterior elongation in area 5 
is slashed by the marginal periproct. Adapical surface evenly 
convex, slightly flattened at apex. Ambitus rounded, ill-defined : 
there is no “ posterior surface” (Pl. VII, Fig. 3). Adoral surface 
concave for the inner two-thirds; ambulacra hardly impressed, 
interambulacra very slightly tumid, except 5, which forms a marked 
protuberance below (i.e. orad to) the periproct (Pl. VII, Figs. 3 and 4). 
Peristome not very clearly shown (? imperfect), slightly longer 
than broad, apparently oblique in the direction 3-I, but with an 
asymmetrical bay in IT (? due to fracture). Invagination faint, 
being a continuation of the curvature of the adoral surface. 
Periproct (Pl. VII, Fig. 3) large, strictly marginal, forming a marked 
indentation in the ambital outline, as seen from either surface. 
Rounded adorally, greatest width on ambitus, sharply acuminate 
adapically. 
Apical system (Pl. VII, Fig. 5) small, approximately central, 
truncating the apex of the test. Four relatively large genital pores 
arranged in a trapezium with the posterior pair more widely 
separated (3 and 4 lie parallel to the antero-posterior axis, 1 and 2 
make an angle therewith). Plates arranged as in Pyrina or Conulus, 
but contiguous posterior oculars subequal. 
Ambulacra (PI. VII, Fig. 7) similar, with simple pores throughout ; 
Tand V form a recognizable “ bivium ”. Adradial sutures practically 
straight, sharply divergent adapically, subparallel on and above the 
ambitus, gently convergent adorally. Areas hardly inferior in 
width to interambulacra on peristome-margin. Plate-structure 
typically Pyrinid, wherever visible. Pore-fields very narrow 
adapically, with close-set, uniserial pore-pairs in faintly sunken 
peripodia ; widening rapidly just below the ambitus, with strongly 
triserial pore-pairs (? without peripodia) over almost all the adoral. 
surface. Pores uniformly minute. Major ornament in each column 
consisting of an external primordial row of tubercles continuous 
from peristome to apex (much reduced in size towards the ambitus) ; 
a second row (with very large tubercles adorally) extending from the 
fifth primordial tubercle to a point rather nearer the ambitus than 
the apex; and a third, internal, row (large below the ambitus) 
extending from the middle of the adoral surface to a point 
considerably above that reached by the second, with a few scattered 
members quite near the apex. Rows one and three are situated on 
the largest plates of the triad-groups, row two on the smaller 
primaries. 
Interambulacra (PI. VII, Fig. 7) slightly dissimilar, 2 and 3 being a 
very little less wide than 1 and 4; while 5 is distinctly narrower 
(though not carinate) and strongly inflated below the periproct. 
Plates remarkably even in height throughout, those around the 
ambitus being very slightly lower than the rest. Major ornament in 
