74 Triassic Echinoderms of Bakony. 
shown by specimen c (figs. 124, 130), pass over the edges of the interambulacral 
plates, which are bevelled on the inner surface, and form thereon a series of denticles, 
which gradually die away on the inner surface of the interambulacrals. Where the — 
denticles die out is a slight depression, followed by a slight ridge parallel with the 
adradial margin. The spaces between the denticles are twice as wide as the denticles. 
Comparison of all the available specimens shows a change in the suture as it passes 
from the adapical to the adoral region (fig. 129). In the adapical and ambital regions 
the bevel on the inner surface of the interambulacral margin slopes gently so as to 
form a sharp edge, and on this bevelled surface lie the denticles, the spaces between 
them corresponding, as above described, with the concave scollops of the extreme edge. 
Towards the adoral end the edge gradually thickens, and a portion of the bevelled 
surface comes to lie at a much steeper slope, until, at the extreme adoral end, the 
suture is at right angles to the outer surface and is considerably thickened so as 
to form a relatively broad ridged surface inosculating with the ambulacrals. Thus 
the regions of the suture from apical to oral present a series similar to that regarded 
by Prof. Dorpertemn (1887 N. Jahrb. f: Mineral.) as the historical succession, and 
represented in his figures 5 (7. subsimilis), 6 (T. liagora), 8 (Rhabdocidaris anglo- 
suevica), 10 (Cidaris Thouarsi). This fact is not necessarily opposed to Prof. 
DoEDERLEIN’S hypothesis, but it shows the danger of basing general statements on 
isolated fragments, and suggests the exercise of great care in using the bevel of the 
suture as a means of distinguishing Triadocidaris from Microcidaris. One result of 
this structure is to give flexibility to the adapical region and rigidity to the adoral, 
this rigidity being, no doubt, correlated with the development of a peristomial frame- 
work. The inner surface of the interambulacrum, as seen for instance in c (fig. 
130) and d, does in fact present a thickening along the border of the peristome, rising 
into an elevation at each adradial angle. Confirmation of this suggested rigidity is 
presented by the partly crushed small specimen of 7. subsimilis (E8547). Here 
the plates of the adoral region of the test retain their normal position, while in the 
upper half the interambulacra are markedly thrust over the ambulacra as far as to 
the inner pores. 
So far as the material in the British Museum allows one to judge, all the species 
of Triadocidaris present a structure like that just described. It may be thought that 
the admirable and suggestive papers by Prof. DorpeRLein render this minute account 
unnecessary. It must therefore be pointed out that the present account differs from 
that of my eminent predecessor, not merely in the description of structural variation 
in different regions of the suture, but in the essential details as to the articulation 
of the bevelled surfaces. While the appearances presented by those surfaces when 
separated suggested to Prof. DorDERLEIN that each denticle of the interambulacrum 
fitted into a groove or depression in the middle of the corresponding ambulacral, 
and though such an interpretation affords a needed and a plausible explanation of 
the grooves in question, still none of the united specimens before me, whatever its 
degree of overthrust, confirms that inference; on the contrary, the denticles, or the 
points of the scollops in which they end, invariably rest in the other series of grooves, 
those, namely, between the ambulacrals. Thus at a certain degree of overthrust the 
concavity of the scollop corresponds with the outer margin of the outer pore, leaving 
a free exit for the podium. Should the overthrust proceed further, as one can see 
that it occasionally did, then the pore would be gradually closed, allowing time for 
