746 Ww. P. SLADEN ON LEPIDODISCUS LEBOURI FROM 
same side, curtailed, so that they form together a wedge-shaped 
pair; this requires a reciprocal modification of the following plate, 
or next two plates, as the case may be. The apex of the wedges, 
whether single plates or pairs, not unfrequently reaches nearly to 
the opposite side of the ray, the corresponding companion plate 
being in consequence reduced to a quite insignificant secondary 
plate or scale. Sometimes the alternation of wedge-shaped plates, 
or pairs of plates, is continued, in the manner above indicated, for 
several successive series, without a break, along the ray (see fig. 3). 
The broad, band-like plates occur at intervals between the others, 
and in one or more of the rays a series of three or four follow in 
succession immediately after the irregular apical plates, and form 
the first of the brachial series at the inner extremity of the ray. 
In one of the rays, on the outer portion which surrounds the peri- 
phery of the disk, there is a large and uninterrupted series of simple 
plates; but whether these occur in the same portion of every ray 
it is impossible to say, in consequence of the damage which our 
specimen has suffered. Hach of these broad plates has generally 
both the proximal and the distal margins somewhat bevelled off at 
either side, and the extremities more or less rounded, thereby 
leaving a narrow, triangular space or aperture at the extremities of 
the suture between any two plates and close up to the junction 
with the adjacent imbricating interbrachial area (see fig. 4). The 
lateral ends of these plates are also a little thickened ; and at a short 
distance from the same the plate is somewhat rapidly, although only 
slightly, arched upwards, thus conforming to a dorsal convexity of 
the ray—a character which the wedge-shaped plates also exhibit in 
a greater or less degree. 
The interbrachial areas are narrow and irregularly subpetaloid, 
with the inner or arm-angle well rounded; they are covered with 
large subrhomboidal plates, which are strongly imbricated inwards— 
that is to say, an outer plate overlaps the margin of its more internal 
neighbour. In relative proportion the length of a plate is usually 
equal to, or sometimes greater than, the breadth, one of the largest 
measuring 0-1 of aninch. The free angle is slightly rounded, and 
this becomes more pronounced as the plates approach the periphery, 
where they are also proportionally broader. The plates immediately 
within the marginal portion of the ray are somewhat smaller and 
more squamiform than the rest; whilst those on the outer side, 
which form the margin of the disk, become broad short scales, quite 
different in facies from the diamond-shaped plates which constitute 
the general tessellation of the interradium. It seems probable that 
this marginal series of scales was imbricated outwardly, that is, in 
the opposite direction to the inner series; but, unfortunately, very 
little of this portion of the fossil remains. Along the line of junction © 
of the interradia with the raised rays are traces of a series of small 
seale-like plates, which imbricate or abut upon the flat ends of the 
plates of the brachial series; these, however, can only be detected 
here and there, owing to the condition in which the fossil is pre- 
served, and their obscurity is explained by the supposition that 
