XI. Balanocrinus. 389 
perhaps in the newly-formed columnals of the proximal 
region in older stages, the arrangement may depart less 
from that of Isocrinus. This is shown in the following 
figures of de Loriol’s plates in ‘ Paléontologie Francaise’ : 
pl. clxxxiil. figs. 66, 70; pl. clxxxvi. fig. 114; and toa less 
extent in others. On the other hand, columnals of Jsocrinus 
may sometimes approach the Balanocrinus plan. 
Description of B. didactylus.—Turning now to the specimens 
from Biarritz, we note that the Normal Joint-face is of 
markedly Balanocrinus type. There is some variation, but in 
a fully-grown columnal of 9 mm. diameter, from a median 
or distal region of approximately circular section (E 21948), 
the arrangement is as follows (Fig. 1) :— 
Outline subcircular, being slightly flattened on the inter- 
radii and cut into on the radii where the radial pore enters. 
Peripheral crenellae in each sector about 10, at right angles 
to the periphery, unequally spaced and of unequal thickness, 
those nearer the radii being stouter and sometimes double, 
_ length about 1 mm., but longer towards the radii (1°3 mm.) 
and rather shorter on the interradius, sometimes slightly 
curved or waved in their course, confluent externally, so that 
the suture is not crenelate. Occasionally a slight marginal 
rebate. Radial ridge-groups: about 3 outer pairs of adradial 
crenellae, continuing the slope of the adjacent peripheral 
crenellae, are gable-shaped, but rapidly decrease in size, and 
are succeeded by a series of minute crenellae, which, when 
visible, are in opposed pairs, but which generally unite 
radially to form a couple of ridges leading to the central 
area. These ridges are separated by the radial canal ; 
towards the middle of their course they thicken and may be 
wider apart, attaining a width over all of 1 mm.; towards 
the centre they taper and again draw together, generally 
meeting with one another and with their neighbours, so as 
to form a narrow ring round the lumen, which ring corre- 
sponds to the central area. Lumen subpentagonal, diameter 
0-4 mm. Floors depressed or even excavate, bounded 
adradially by concave curves (due to the widening of the 
radial ridge-groups). 
It follows without doubt from this description that the 
species is rightly referred to Balanocrinus. 
Such variations as occur in the normal joint-face, other 
than those already indicated, are correlated with the variation 
~ in Transverse Section. This assumes all the forms possible 
to a Pentacrinid stem, with this proviso—that, as in Balano- 
