258 
Triassic Echinoderms of Bukony. 
markings, which Loven merely suggested might be gonopores. The disappearance 
of primary tubercles from the supra-ambital region is no rare phenomenon, and may 
be seen even in some Upper Cretaceous Cidarids; moreover the restriction of these 
tubercles is so much more marked in Tiarechinus than in Pygmaeocidaris that this 
can scarcely be considered as a point of resembiance. In any case their distribution 
depends to a large extent on that of the plates supporting them, and here vve must 
recognise a considerable difference between Tiarechinus with its four interambulac- 
rals and Pygmaeocidaris with fifteen. Lysechmus, if Gregory’s interpretation be 
correct, has nine interambulacrals, but Doederlein does not consider it so compa- 
rable with Pygmaeocidaris. The really important resembiance between Tiarechinus 
and Pygmaeocidaris seems then to lie in the presence of unpaired median inter¬ 
ambulacrals. In Tiarechinus the proximal interambulacral is retained, and Supports 
one other unpaired plate reaching up to the genital plate. In Lysechinus there are 
said to be two other unpaired plates, but they are separated from the proximal 
plate by the ingrowth of the first plates of the a and b columns. In Pygmaeo¬ 
cidaris the proximal interambulacral, which bears a tubercle, is succeeded by a 
small unpaired plate without a tubercle, and this again by a large tuberculiferous 
median plate. Now, while the mere possession of unpaired plates is common to the 
Tiarechinidae and to Pygmaeocidaris , there are in the disposition of those plates 
differences that seem of some importance. The proximal interambulacral in Pygmaeo¬ 
cidaris does not Stretch across the whole interambulacrum as in Tiarechinidae, 
but lies between the gill-notches, which are excavated in the first paired interambu¬ 
lacrals. The median interambulacral series does not reach the apical System as in 
Tiarechinidae, but is separated from it by three pairs of interambulacrals. The two 
upper plates of the median series do not, as in Tiarechinidae, ränge horizontally 
with the adjacent plates of the paired series, but alternate with them. The sutures 
between the successive median plates are not horizontal as in Tiarechinidae, but 
slanting; and a still more notable departure from bilateral symmetry is manifested 
by the uppermost plate shown in Doederlein’s fig. 37, and described by him as 
«nach der einen Seite mehr verbreitert als nach der anderen und erreicht dort mit 
einer Spitze gerade noch das Ambulacralfeld, indem sie die beiden paarigen Platten 
dieser Seite auseinander schiebt». The plate in question may very well be the 
4th of the a column, counted from the adapical end. If the interambulacrum be 
reconstructed with this plate in the normal position, it will be necessary to Stretch 
b 4 across so as to fit in between a 4 and a 5. It will then be seen that the 
median plate below it, which bears no tubercle, can naturally be interpreted as 
b 5, similarly squeezed into the middle line, but retaining its position between a 5 
and a 6. Below a 6 follows b 6 in its natural position above the proximal inter¬ 
radial plate. The other genera of Arbaciidae that retain the proximal or primary 
median interambulacral in the neanic or the adult stages show no trace of the upper 
series of median plates, but examination of the very clear figures of Arbacia and 
Dialithocidaris published by Mr. Agassiz (1904 «Panamie Deep Sea Echini», pl. liv, 
ff. 2, 5, 6; pl. xxiii, ff. 1, 4, 5, 6) shows how the interambulacrals are already 
very oblique and how further pressure in an adoral direction along the adradial 
tracts would tend to force the inner tuberculiferous portion of some plates into a 
more interradial position. The extreme widening of the interambulacrum in this 
manner just at the ambitus in Pygmaeocidaris prionigera is in harmony with the 
