Echinoid Radioles, Cidaris similis. 189 
nounced on one (? adapical) side; collerette distinct, striate; annulus sharp, pro- 
minent, striate; acetabular margin prominent, smooth. 
The Holotype must be one of the specimens shown in the figures repro- 
duced by Desor, and I therefore select the original of MUtnsrer’s f. 24 b, since it 
shows the base. It comes from St. Cassian, and is preserved in the Palaeontological 
Museum, Munich. 
Notes on St. Cassian material. — No complete radiole was known to 
Munster, and none is known to me. The longest fragment in the British Museum 
(Kuieste Collection) measures 18°5 mm., with a greatest transverse diameter of 
2°4 mm. including the pustules, and a greatest sagittal diameter of 2,1 mm. It is 
the proximal end of a radiole, and the greatest thickness is at about two-thirds of 
its length. The diameter of the annulus is 1°9 mm.; the outer diameter of the 
acetabulum, 1:2 mm. The distal end of a radiole in the same collection has a 
length of 10 mm., and tapers from 1°7 to 0°6 mm. in transverse diameter, from 
15 to 1°0 mm. in sagittal diameter; the actual extremity was broken off apparently 
during life, and the broken edges rounded and slightly thickened. Calculation from 
these data gives 35 mm. as the minimum length admissible for the former fragment. 
Fragments having, as a few have, a transverse diameter of 3 mm., may therefore 
have come from radioles at least 44 mm. long. Mdnster’s restoration seems to 
suggest a radiole rather longer than this. The majority, no doubt, were smaller. 
The small size of the acetabulum implies a small mamelon, and probably a small test. 
The regularity of the ridges is a conspicuous feature. Monsier gives their 
number as from 8 to 10. The first specimen measured above has 8 ridges and a 
broad smooth back. A narrower proximal fragment has 5 ridges, with an incipient 
sixth. The largest proximal fragment and the distal fragment previously mentioned 
have each 7 ridges with an eighth obsolescent or incipient. The number 10 seems 
to be rare and confined to such radioles as have ridges on the back. 
The pustules usually form spinelets with a rake distalwards. The longest 
proximal fragment has 19 in one of its lateral rows, occupying 15.5 mm. of the 
shaft. The lateral rows are generally the more complete and regular, and their 
pustules have a slight dorso-ventral compression; the intervening rows on the 
supposed adapical face are more pronounced but less regular. 
Between the rows and on the frequently smooth back (? adoral face) is seen 
a very fine longitudinal striation, due to the outcropping radiate septa. The trabe- 
culae are also clearly seen. 
The striae of the collerette are rather coarser than those of the shaft. 
The annulus is, as a rule, cut to a sharp edge by a flat platform on the 
distal side, and a slope continuous with the conical base on the proximal side. 
The micro-structure (Pl. XV, fig. 444) consists of an axial complex, occupying 
about one-seventh the total diameter; and of radiate septa, very closely set, and 
joined by obscure trabeculae. These septa constantly diverge, fork, and re-unite, 
so that all have a peculiar wavy course. Towards the pustules they fan out and 
fork rapidly, at the same time thickening. The trabeculae seem rather stout near 
the periphery, but are very irregular; it is very hard to distinguish them at all 
in the inner parts of the section. Here the septa often seem broken into short 
isolated blocks, an appearance probably due to their perforation. Near the periphery 
the septa are about 70 to the millimetre, a measurement that also applies to the 
