Echinoid Tests, Cidaroids. 
75 
the withdrawal of the podium ; and eventually the overthrust would be checked by 
two structures — the meridional ridge on the inner surface of the interambulacrum, 
and the elevation or granule between the inner and outer pores. If this interpretation 
be admitted, what, it may be asked, is the meaning of the groove or depression 
in the middle of each ambulacral ? Perhaps it was a fossa for the attachment of a 
muscle or ligament; perhaps, together with the groove from the other pore, it was 
a furrow for fibrils of the peripheral nervous System. 1 
I should hesitate to oppose this interpretation to that of Prof. Doederlein, 
were it not that his account is inconsistent with his own hypothesis of the evolution 
of the modern type of suture. On that hypothesis «die Rippen, die auf die Rand¬ 
fläche beschränkt bleiben, verursachen die zickzackförmige Knickung». In the zigzag 
suture of a recent Cidarid, the projections of the interambulacrum alternate with 
the ambulacrals, and it is obviously the meaning of Prof. Doederlein that these 
projections are homologous with the ridges or denticles. This homology may in 
fact be regarded as proved by the change observed in the different regions of a 
single suture in T. persimilis. But if so, the denticles must alternate with the 
ambulacrals instead of coinciding with them. And the Observation of united spec- 
imens, as opposed to Prof. Doederlein’s inference from isolated fragments, shows 
that they do alternate. 
A further argument against Prof. Doederlein’s view is presented by various 
species of other genera (e. g. Echinocrinus = Archaeocidaris, as observed by 
A. Tornquist, N. Jahrb. f. Mineral. 1896, II, p. 42), in which, although the inter 
ambulacral margin is denticulate, there are no grooves on the ambulacrals. 
In T. persimilis the other margins of the interambulacrals are not exposed as 
a rule, but the plates are broken across at other levels, a fact which shows that 
the union was close and rigid. Only in specimens s and t can any part of a transverse 
sutural surface be detected; it is vertical or very slightly bevelled, with a faint 
median depression. 
Relations of the Species.—In T. venusta the scrobicular ring consists 
of only 7-—8 small secondaries, and the ambulacra, according to Münster, are quite 
straight. T. Liagora, which has similar Ornament, has a larger number of inter¬ 
ambulacrals in a column, with the result that the scrobicules are contiguous, with 
scrobicular tubercles reduced in size, or are even confluent. In T. Stiessi a further 
increase in the number of interambulacrals results in more elliptical and more 
confluent scrobicules. In T. subnobilis also the scrobicules are confluent and have 
no definite ring of tubercles. The remaining species T. subsimilis is more closely 
allied to T. persimilis than are any of the preceding, and the two rriight perhaps 
be considered as local races. As has been shown in detail, T. persimilis is smaller, 
more depressed, with perhaps fewer interambulacrals in column, and, consequently, 
with interambulacrals relatively higher, and much larger intertubercular tracts; the 
mamelon of the main tubercles is narrower and less prominent, the secondary 
tubercles are more numerous and those of the scrobicular ring less pronounced; 
the ambulacra differ in minute details, perhaps of merely individual significance. 
Taking the major differences, we may place the species of Triadocidaris 
hitherto described, excluding T. vemista, which Lambert refers to Microcidaris, in 
1 See Pkouho «Recherches sur le Dorocidaris», Arch Zool. Exper. (2j V, p. 245, pl. XIV, fig. 2, sn. 
