534 CETACEA. 



appear on the seventh lumbar, in which also the metapophyses are suddenly- 

 diminished in breadth, being reduced to narrow pointed processes, embracing the 

 bases of the spinous processes of the vertebrae. This change takes place gradually 

 from the first lumbar in which the zygapophyses and metapophyses are as in the 

 last dorsal. The spinous processes are more developed than in the dorsal region, 

 and increase in size to the eighth lumbar, which has the highest process in the 

 whole vertebral column. The bodies are much larger than those of the dorsal 

 segments, and go on increasing to the last, which is about the largest in the 

 column, with the exception of the first two or three caudals, which slightly exceed 

 it in size. They are all broader than long, and broader in their anterior than in 

 their posterior surfaces. The articular surfaces are transversely oval. Their under 

 surfaces in the first five have the same characters as the dorsals, but in the last 

 three the under surface presents two hypapophysial ridges in its centre. The 

 neural canal becomes much laterally compressed from the first to the last lumbar. 

 In the former it is regularly triangular, whereas in the latter the height is nearly 

 twice as much as the breadth, due to the pedicles gradually approaching each 

 other by occupying a higher place on the bodies. 



The ten dorsal, when in position, equal the length of seven lumbar vertebrae. 



Caudal vertehrce. — These are distinguished nearly throughout the whole of their 

 extent by chevron bones, which are absent only in the last three intervertebral 

 interspaces.^ The two halves of the first chevron bone do not usually unite inferiorly 

 in the mesial line, and one-half occasionally amalgamates with its fellow behind 

 it, the two halves of the two bones of the opposite side remaining distinct from 

 each other. The third chevron has the most vertical extension, but it is antero- 

 posteriorly narrower than those behind it as far as the twelfth. The chevron bones 

 gradually diminish in size to the fourteenth, beyond which they rapidly decrease. 

 The processes for their attachment are most prominent in the fifth caudal, but they 

 can be traced backwards to the very last, bearing chevron bones, that is, if the two 

 median ridges which define the outer margins of the depressions on the under surface 

 of the bodies into which the foramina of the branches of the caudal artery open can 

 be regarded as serially homologous with these processes ; but if not, they occur 

 unmistakably as far back as the fourteenth. On the side of the bodies of the fifth, 

 sixth, and seventh caudals, below the transverse processes, a small process appears on 

 the anterior side of the strongly marked oblique groove which passes forwards and 

 downwards from the posterior border of the transverse processes, and along which 

 the branches of the ventral vessels of the tail are transmitted. They become 

 double in the seventh caudal, one being on either side of the groove, and they become 

 more distinct and attain their greatest development in the tenth and eleventh caudal, 

 beyond which they unite with each other and form a strong ridge near the lower 

 border of the lateral surfaces of the twelfth to the fifteenth vertebrae. The lower 



1 Professor Flower says that chevron bones cease to be developed after the caudal vertebrse enter the laterally 

 expanded portion of the tail, but in Orcella the chevron bones occur up to the very last vertebra, and in Platanista, 

 with the exception of the terminal three, they are present in all. 



