56 ON THE ECHINODEEMATA OF THE
sides of the shields are rather straight and are at right angles to the aboral margin for
some space, and then they slope inwards to the oral point. An accessory plate exists,
in some, between the oral angle of the mouth-shield and the side mouth-shields. The
madreporic plate is cribriform.
The side mouth-shields are rather large, do not unite closely within, are long and
rectangular, being widest at the side angle of the mouth-shield, where they are more or
less pointed, curved, and rounded off,
The generative slits are short and linear; the edges are close together, and have on
them small, flat, rounded spinules, sixteen or more on each; they are distant from the
mouth-shields and from the margin of the disk; and a series of fine scales passes outwards
from their distal end, by the side of the arm, to the margin. Other minute scales
are in a patch on the oral side of the slits.
The jaws are rather long, stout, tumid and bossed, and form rather a sharp angle ;
and the lower edge of the jaw-plate is broad and stout; the angular spaces are wide and
large; and the tentacles are very well developed.
There are mouth-papillse and tooth-papillse; and the teeth are in a double vertical
series with some accessory knobs, so that they resemble large tooth-papillse.
The mouth-papillae are numerous, fourteen or fifteen to each angle, small, much
joined together at their bases, irregular in size, shape, and number, short, and never very
broad. At the apex of the angle, within the distinct jaw-plate, there are three principal,
and one or more smaller, lowest tooth-papillse. No satisfactory distinction, except that
of position, can be made between these tooth-papillse and mouth-papillse. Above the
three or more lowest tooth-papillse, the others are in a crowded vertical series. They are
most numerous and small on either side; and there are six or seven pairs of large, long,
pointed and irregular-shaped ones in the midst, and reaching up the jaw-plate to its upper
end, occupying the position of the teeth. The side of the jaws, close to the jaw-plate,
is occasionally covered with small and close papillse; and there is a small accessory
papilla close to and at the side-of the uppermost large ones. The upper part of
the jaw beneath the stomach is stout and tumid, and the jaw-plate is large and well
developed.
There are two rounded knobs on the side of the jaw, above the attachment of the
mouth-papillse, which are in relation with the upper tentacle; and the lower tentacle
of the angle has five or six short unequal-sized tentacle-scales, forming, with several
accessory scales, an obliquely placed curved wedge-shaped mass within the first lower
arm-plate and on either side of its oral margin.
The lower arm-plates, there being six or seven within the disk, are mostly very broad
and very short; further out they are small and triangular, with an aboral projection.
They form but a small portion of the lower surface of the arm. The side arm-plates
meet below, from the first to the last, giving a broad and comparatively flat under
surface.
The first lower arm-plate is unlike the others in shape, and it is elliptical in
outline and much broader than long; the second, longer and very much broader than
the first, is somewhat rectangular; its sides are slightly incurved; and there is a
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central angular process on the distal and proximal edges, from which there is a reentering
curve on either side to the lateral angles of the sides of the plate.
The third lower arm-plate is very broad, extending across the arm; it is short, the
relation', of length to breadth being one to three ; the sides are incurved for the.
tentacle, and are slightly convex towards their distal angle. There is an angular
process or cusp on the broad oral margin, and a smaller one on the aboral; and there
is a reentering curve on both sides of the. processes, giving a very elegant outline.
The fourth lower arm-plate is as broad as the third; but it is shorter, and the proximal
angular process is more decided than that on the distal edge. The next plate has the
-general shape, but is shorter, and the proximal angle is more pronounced. From this
plate to the end of the arm the others narrow more and more, become angular at the
sides and more or less triangular as a whole, and are broader without than within.
There is a projection in the median line on the aboral margin, and a reentering curve on
either side to the lateral angles; and the proximal angular process has faintlyreentering
curves on either side of it. Far out and towards the tip of the arm, the lower plates
become more quadrangular or diamond-shaped; they are small, broader than long; and
there is an angular process without and within. At the tip the minute lower arm-plates
have the distal edge curved, and they are angular orally.
The first lower arm-plate is separated from the side mouth-shields by several scales
or plates, which are continuous with the base of the tentacle-scales already mentioned
as being within and at the sides of the plate.
- The second lower arm-plate has two or three small tentacle-scales on it. In
some arms they are fused into one; or there may be several minute accessory scales
present.
The third plate has the same number and accessory arrangement-; and they are
seen, more or less modified, in the fourth and fifth. The other lower arm-plates have
no tentacle-scales.
The side arm-plates form much of the lower surface of the arms, and also the
greater part of the sides in mid arm and towards the tip. All unite with their fellows
along the median line below; and all are convex from side to side. The first, just
touching its fellow in the median line, is oval in outline, broader than long; it supports
five short, close tentacle-scales on a curved base; and the tentacular opening is large
and circular, having a rim of membrane. The second is broader than the first,
touches its fellow, and has four- tentacle-scales, and sometimes a fifth or a small
accessory one.
The third side arm-plate, still broader than long, and not much longer than the
second touches its fellow by a longitudinal short and straight line. The margin without
is curved boldly, and within very slightly. Like the others it is flat below ; and it has
three tentacle-scales, the outer one being subspiniform. The fourth, still broader, is
not longer; and its outer end is large and supports three tentacle-scales, one of which is
sometimes wanting. The fifth plate the broadest, is short, and is narrow towards the
median line, where it has two small tentacle-scales close together; and there is a spinule
external to them, and sometimes a second.