Echinoid Radioles, Cidaris parastaditera. 211 
As the preceding description of outer form will have shown, the normal limits 
of variation are considerable. There are, however, yet other specimens most prob- 
ably belonging to this species, but departing still further from the norm. 
A radiole from Quarry near Cutting i, 4.7 mm. long, subclaviform, with greatest 
diameters 1°77 and 1°8 mm, diameter at annulus 08 mm., has a smooth handle, 
distal pustules well developed, few, and rounded, beginning lower down on the 
adapical face. This probably comes from the circumapical region. 
From Cutting IV is a fragment of triangular section; the adoral face, which 
is rounded proximally, becomes almost flat distally and slightly ridged, while the 
adoral face rises by flat sides into a median angle, the pustules being stronger on 
the angles; the main axis is slightly curved to one side, as well as being con- 
cave on its adapical face; greatest diameters 2°1 and 2:9 mm. 
From the Quarry near Cutting Lis a radiole lying on matrix, with the adoral face 
exposed (Pl. XIII, fig. 392); this is slightly lanceolate in outline, and is quite smooth 
except for a faint ridge near the distal end; as first seen, it was peculiar in having 
two prominent thorn-like pustules projecting distalwards about two-thirds of the way 
down the right side, and a single similar pustule on the left; further removal of 
the matrix showed that these apparently isolated thorns were merely members of 
the lateral series, and that pustulate ridges were well-developed on the adoral face. 
Length, 10°7 mm. ; greatest transverse diameter 2°99 mm.; diameter at annulus 1 1 mm. 
The micro-structure has been studied in three sections of radioles from beds 
a, b of Cutting IV, but only one of these could be deciphered This was a task of 
much difficulty, owing in part to the poor preservation of the section, but still more 
to its very unusual structure. The appearances that can be detected have been 
most carefully drawn by Mr. G. T. Gwrtiam (Pl. XVI, fig. 447.), who has adequately 
grasped the essential facts of this most obscure structure. There seems to be a 
central complex, possibly breaking down into a lumen. A darker patch indicating 
the presence of this structure can often be detected in broken ends. Towards the 
periphery septa are fairly distinct in places, and seem to fan out where the ridges 
occur; this is often the case with the ordinary radiating septa in pustulate radioles 
of other Triassic species. In the region between the outer layer and the axial 
complex, the meshes of the stereom are much finer, and are not composed of 
radiating septa and trabeculae. The appearance is as though the outer septa of 
the several ridge-systems continued inwards, curving in the same direction, that is 
to say, continuing the ribs of the fan, and therefore crossing one another at an 
acute angle. It further appears that under the grooves between the ridges the septa 
fan centripetally, and these also cross the septa previously mentioned. No guiding 
lines of any kind can be distinguished, and the stereom in the intermediate region 
is extraordinarily dense. On the periphery the septa are about 80 to the millimetre. 
Relations of the Species. — There are several Cassian species to 
which, in one or other of its many forms, Cidaris parastadifera presents a resem- 
blance. 
As already mentioned under Cidaris similis, it is often hard to distinguish 
between that species and the normal baculiform radioles of C. parastadifera, so 
far as outer form is concerned. The chief difference appears to lie in the absence of 
longitudinal striation from the annulus, collerette, and shaft of C. parastadifera. 
This difference doubtless depends on the greater fineness of the micro-structure, in 
14° 
