32 



DISCOVERY REPORTS 



bones. Two longitudinal ridges with concave margin join the anterior to the posterior facets. The 

 concavity is ill defined at the anterior end of the series, and is correlated with the lesser prominence 

 of the facets themselves ; but going tailwards with the greater development of the articular surfaces 

 and the shortening of the length of the mass of the centrum, the emargination becomes increasingly 

 pronounced until on the ninth (in both the South Australian and the Discovery specimens) a foramen 

 is enclosed. 



CHEVRON BONES (Fig. 7) 



The nine chevron bones figured are an incomplete series; at least one is considered to be wanting. 

 However, those remaining give an adequate idea of the form these bones assume in H. planifrons. 



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 at some 

 distance from the tubercle. In the seventh the tubercle and capitulum approximate again and the 

 eighth, as just stated, has no capitulum. 



