. Fig. 7. Chevrons. ( x \.) Only one side of the first chevron is present, a slender lamina of bone which has no evidence of having been fused to the element of the other side. The second chevron, a single bone, has a broad, short, spinous process with obliquely rounded ventral margin. The third has the spinous process greatly elongated with rounded antero-ventral margin, and with hinder and ventral margins meeting at roughly a right angle. From the third tailwards there is a progressive diminution in the spinous process length and a reduction in size of the bone as a whole, in the last of the
. Fig. 7. Chevrons. ( x \.) Only one side of the first chevron is present, a slender lamina of bone which has no evidence of having been fused to the element of the other side. The second chevron, a single bone, has a broad, short, spinous process with obliquely rounded ventral margin. The third has the spinous process greatly elongated with rounded antero-ventral margin, and with hinder and ventral margins meeting at roughly a right angle. From the third tailwards there is a progressive diminution in the spinous process length and a reduction in size of the bone as a whole, in the last of the series the spinous process being only about one-half as long as it is wide. The chevrons show no distinctive difference from those of the South Australian specimen or of H. rostratus. RIBS (Fig. 8) The Discovery H. platiifrons has eight ribs on each side, in this number agreeing with the La Plata Museum specimens. The South Australian specimen has nine pairs of ribs, the ninth pair being small, asymmetrical and obviously vestigial. H. rostratus normally has nine pairs of ribs also, but at least one specimen in the British Museum collection has only eight pairs. In the Discovery specimen the first pair of ribs is short, broad, flattened and with sternal end directed at a slight angle forward from the remainder of the shaft of the bone. The second rib is moderately broad, more elongated than the first and without forward trend of the distal end. The third to the sixth are similar to each other, long, slender and subequal in length. In the seventh, shortening of the shaft has become pronounced, but otherwise the essential features of the four preceding ribs are maintained. The eighth is still shorter, and in the absence of a capitular portion is distinguished from all the ribs that precede it. The first seven ribs have the capitulum defined to a greater or lesser degree. In the first the capitulum and tubercle are almost confluent, in the following five the capitulum is situated
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Keywords: ., bookcentury1900, booksubjectocean, booksubjectscientificexpediti
