Echinoid Radioles, Cidaris dorsata. 181 
In a small shaft (6), 10°3 mm. long, with greatest diameters 64 mm. and 
41 mm., the pustules are much flattened, as though sharing the dorso-ventral 
compression of the shaft; they project in a proximal direction, contrary to the usual 
adpressed spinelets of this mutation, though the feature is noted in the proximal 
region of the adapical face in a few other specimens. 
Shafts of normal pyriform shape are represented by specimens e and f, which 
are rolled and fragmentary. 
The pyriform shape with the first signs of distal excavation is well shown in 
the shaft g (figs. 314—316). This, which seems to be broken off just above the 
collerette has a length of 25°l| mm. and greatest diameters of 11°6 and 11°0 mm. 
In the proximal region the pustules are low, especially on the adoral face (fig. 314), 
and are often hollowed at the ends, as in some specimens of C. alata poculiformis, 
or even turned into slight cavities. In the middle and distal regions the pustules 
gradually become more prominent and directed distalwards. In the middle region 
about 3 pustules of the adoral and 2 of the adapical face occupy a width of 
2° mm. On the rounded distal end (fig. 315) the pustules are again smaller and 
shorter and are irregularly distributed in such a way as to leave bare patches. On 
both the adoral and adapical sides of the distal end is an irregular depression, 
unsymmetrically placed, and containing some pustules. 
The slight cavities at the proximal end of the shaft in specimen g agree in 
position with the far more marked holes found in Cidarites foratus Qurnst. The 
latter, however, appear to be due to some boring animal (fig. 334), whereas the 
cavities in specimen g are probably due to slight erosion and perhaps natural 
resorption of the proximal pustules during life; they have been noticed only in this 
large radiole and can scarcely be seen without a lens. There is a more constant 
resemblance between this mutation and Cidarites foratus, namely the prominence of 
the pustules (fig. 335). In the case of C. foratus I regard that feature as hyper- 
trophy in response to the stimulus of the boring animal, and on that view the species 
is a synonym of Cidaris dorsata. Figure 12 e on plate ix of Lauper (1865) does in fact 
represent (though poorly) a specimen referred to C. dorsata but in the foratus con- 
dition. It does not follow that the spinulose pustulation of C. dorsata marginata was 
inherited as an «acquired character» from C. dorsata «forata». 
Smaller radioles of the same general form as g are h, 7, k, 1, m. The regularly 
pyriform 4, which is complete, has a length of 19°4 mm, with greatest diameters 
9°7 and 8'7 mm. In these, as well as in g, the pustules often run in oblique lines, 
each pustule lying at the intersection of two lines. One of these lines, starting 
from the side, near the proximal end, trends distalwards to the middle of the 
adapical face where it connects with the corresponding line from the other side. 
Thus these two lines form a wide parabolic curve convex distalwards. The lines 
crossing the limbs of this, pass on to the adoral surface, where they meet in a 
parabolic curve of similar position but narrower. 
The micro-structure of such radioles is described and compared with that of 
the norm on p. 174 under C. alata (Pl. XIV, fig. 439). 
In m the base is preserved (fig. 317), and in this radiole the characters of 
the supra-ambital series just begin to be distinct. The sagittal line of the adoral 
face forms a slight concave curve, from the annulus almost to the distal end, where 
it bends suddenly, at little more than a right angle, to the apex (fig. 318). Neither 
