30 DISCOVERY REPORTS 



atlas is also larger than that of//, planifrons. Otherwise the cervical mass is much alike in both species. 

 Such differences as exist between the South Australian and the Discovery specimens may be regarded 

 as coming within the range of individual variation. The former has the lateral process of the atlas 

 fused with the inferior lateral process of the axis, whereas in the Discovery specimen the inferior 

 lateral process of the axis is distinct. Both specimens show a short rugose superior lateral process 

 on the axis, the South Australian specimen having ' an incomplete foramen on the right and complete 

 foramen on the left between it and the inferior lateral process ', whilst the Discovery specimen has 

 this arrangement of foramina transposed. The superior lateral processes of the third to sixth 

 vertebrae are separate and of diminishing size antero-posteriorly in the Discovery specimen. 

 The South Australian specimen has the third ankylosed on the left with that of the preceding 

 cervical. 



The neural arch of the sixth is not completely fused with the arches anterior to it, and fusion is 

 less on the left than on the right side. The corresponding arch in the South Australian specimen 

 appears to be completely fused. There is a strong forward-projecting inferior lateral process on the 

 sbcth vertebra of the Discovery specimen. Hale (1931) does not mention its presence in the South 

 Australian specimen, and his figure shows that the inferior lateral process of the seventh is of con- 

 siderable size and prominence and similar to that of the specimen of H. rostratus used for comparison 

 with the Discovery H. plmiifrons. The inferior lateral process of the seventh in the Discovery 

 H. planifrons is small and inconspicuous. Between it and the superior process is the articular facet 

 for the head of the first rib. The neural arch is free except at the tip, whereas the South Australian 

 specimen has the ' greater part of right side of neural arch free including apex which does not meet 

 the opposite member of the arch '. 



Thoracic vertebrae. The Discovery specimen has eight pairs of ribs and therefore eight thoracic 

 vertebrae. As the South Australian animal had nine pairs of ribs the possibility was considered of the 

 ninth pair in the Discovery specimen having been overlooked. However, this is discounted to some 

 extent by the fact that in the La Plata examples eight, not nine, is the number recorded. The reduction 

 to this number represents the extreme reached in any of the Mammalia. 



The series of thoracic vertebrae in the Discovery Bottlenose commences with one having a slender 

 neural spine, wide neural arch, widely separated zygapophyses, and short metapophyses at the 

 proximal ends of transverse processes, which last are directed downwards and forwards and bear a 

 facet for the tuberculum of the rib. There is a short centrum bearing a postero-lateral facet for the 

 capitulum of the second rib. Proceeding tailwards the neural spines increase in length and width, 

 the neural arches diminish in size, and the zygapophyses are very much reduced. The metapophyses, 

 from being stout and short, are, in the eighth thoracic laminar, almost semicircular in outline and 

 projecting from the anterior edge of the neural arch. The centrum at the end of the series is about 

 double the length of that of the first thoracic. 



The arrangement of the articular facets for the ribs is interesting, and it is unfortunate that the 

 centra of the vertebrae were damaged by the harpoon which killed the animal just at the point where 

 detailed description is most required. However, enough remains to make some sort of interpretation 

 possible. As far back as the fifth thoracic vertebra the articular facets are conspicuous on the postero- 

 lateral edges of the centra. In the sixth vertebra the surface of the centrum on the left side has been 

 obliterated, but the right side which is entire has only the very slightest indication of a facet, whilst 

 the seventh vertebra has a distinct antero-laterally placed facet. It would appear therefore that as 

 far back as the fifth vertebra the capitular articulation is with the rib of the succeeding vertebra, that 

 the sixth is transitional between this arrangement and one in which the capitulum of the rib articulates 

 with the centrum of the same vertebra with which the tuberculum is associated, and that in the seventh 



