Echinoid Tests, Cidaroida. —__ 73 
is more pronounced, and it may have been this that led Monsrer to assert the 
presence of an outer row of tubercles; in that specimen it is continued into a definite 
ridge on the adapical side of each inner pore, which ridge does not continue the ridge 
forming the adapical boundary of the outer pore, but is more adoral in position; thus 
the inner pore is, or appears to be, nearer the oral pole than is its fellow outer pore. 
In the small British Museum specimen of 7. subsimilis (836479) the elevations between 
the two pores of a pair are relatively still more conspicuous, but the outer pores are 
here too much covered by the interambulacra for the relations of the ridges to be made 
out. In the still smaller test E8547, neither the admedian nor the interporal series of 
tubercles are well developed, but the position of the pores is as in the holotype. In all 
specimens the outer pores appear less elongate than in T. persimuilis. 
In both 7. persimilis and T. subsiniilis the ornament of the ambulacra, and 
in the latter species the mutual relations of the two pores of a pair, enable one, 
in the absence of other evidence, to orient any fragment containing ambulacrals. 
It is, indeed, the rule in Echinoidea that the inner pore of a pair is more adoral. 
Thus one learns that Mtnster’s pl. Ill f. 2 is not merely reversed by the mirror, 
but has the adoral end uppermost, while Lause’s drawing of the same specimen 
(pl. IX, f 1 c) has:the adoral end downwards and is not reversed. 
The Union between Ambulacra and Interambulacra. — In the 
ambital and adapical regions the surface of the ambulacrum outside the interporal 
elevation slopes inwards (i. e. towards the interior of the test) so as to pass beneath 
the interambulacrum, as is usual in the genus. Thus the extreme edge of the 
-ambulacrum is only seen when the interambulacrals are removed, and is then found 
to be slightly scolloped, the notches between the scollops coinciding with the grooves 
from the pores, one on the suture between successive ambulacrals and one in the 
middle of each ambulacral. In a@ the edge of the interambulacrum is concavely 
scolloped, each scollop corresponding to an ambulacral; and the projections between 
the scollops lie in the grooves between adjacent ambulacrals. A similar arrangement 
interambulacral ambulacrals 
inner outer 
pore pore 
denticle 
elem g.oove from outer pore 
Xe eee groove from inner pore 
4 
secondary tubercle 4. ~ 
interporal admedian tubercle 
granule and miliary 
Text fig. 10. Diagram to show relations of ambulacrals to interambulacrals in Triadocidaris persimilis. 
The lefthand column of ambulacrals is partly pushed under the interambulacral. 46 diam. 
is seen in T. subsimilis (E8547). The projections between the scollops may or 
may not correspond with secondary interambulacral tubercles. The projections, as 
