. Bulletin - United States National Museum. Science. MONOGRAPH OF THE EXISTING CBINOIDS. 133 outer genital pinnules, like the distal pinnules, lie horizontally in a plane at right angles to the dorsoventral plane of the arm (fig. 1071, pi. 16). The genital and distal pinnules can be folded inward until they almost or quite meet and then laid back along the arm so that they form a covering over the soft ventral surface. Pinnules always occur on the opposite sides of succeeding brachials, so that they alternate regularly all along the arm (fig. 1030, pi. 12). They never occur on the hypozygals o
. Bulletin - United States National Museum. Science. MONOGRAPH OF THE EXISTING CBINOIDS. 133 outer genital pinnules, like the distal pinnules, lie horizontally in a plane at right angles to the dorsoventral plane of the arm (fig. 1071, pi. 16). The genital and distal pinnules can be folded inward until they almost or quite meet and then laid back along the arm so that they form a covering over the soft ventral surface. Pinnules always occur on the opposite sides of succeeding brachials, so that they alternate regularly all along the arm (fig. 1030, pi. 12). They never occur on the hypozygals of syzygial or of sj'narthrial pairs, nor upon axillaries. Usually the pinnules taper gradually, slowly, and evenly from the base to the tip, but in certain species in which the adambulacral plating is highly developed, as in the species of Ptilometrina? and Calometridse, they may end abruptly with a few small segments much as the arms do in the same types (figs. 351-593, pp. 235-303). In viewing a single arm of almost any 10-armed species with the pinnules extended, the elongate oral pinnules ai-e seen to be very conspicuous, and the change from these to the short lower genital pinnules is very abrupt. The following genital pinnules slowly increase in length until the lower distal pinnules are reached, after which the same length is usually maintained until near the end fig. 201.—lateral view of specimen of calometua separata. of the arm, when the length de- creases with more or less rapidity, so that the distal end of the arm is more or less broadly rounded. The slenderness and delicacy of the pinnules and their very con- siderable number give the arms much the appearance of delicate feathers (figs. 262-287, pp. 207, 213. 215). As a general rule, the pinnules of the oligophreate species (figs. 262-280, pp. 207. 213; 283-285. p. 215) are shorter and stouter than those of the species of the Macrophreata (figs. 281, 282. 286, p. 215). Proportionately with the increase in the nu
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