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 vertebra.—These are distinguished nearly throughout the whole of their
extent by chevron bones, which are absent only in the last three intervertebral
interspaces.1 The two halves of the first chevron bone do not usually unite interiorly
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 Slower says that chevron bones cease to he developed after the caudal vertebra enter the laterally
expanded portion of the tail, but in Orcella the chevron bones occur up to the very last vertebra, and in Plataniala,
with the exception of the terminal three, they are present in all.
rounded knobs that appear on the sides of the caudal vertebrae from the sixteenth
to the twenty-second seem to be these processes greatly intensified.
The transverse process of the first caudal is nearly as strongly developed as in
the last lumbar, but in the sixth it has become considerably reduced, and still
more so in the seventh, eighth, and ninth, until in the tenth it is little more
than a ridge pointing anteriorly. The three first transverse processes are directed
backwards, but those succeeding are slightly curved forwards. On the eighth
transverse process the groove for the branch of the caudal artery occurs on the
side of the posterior end of the centrum, but, as it is traced backwards, it is seen
to be moved more and more forwards until it cuts the transverse process near
its middle, as a deep groove. In the fourteenth, the transverse process is reduced
to a mere ridge, which is perforated by the arterial groove. In the vertebrae
behind this, the transverse processes become enlarged and nodular, like the processes
on the inferior aspect of the side already described and from which they are
separated by a longitudinally oblique deep furrow. This enlargement begins to
show itself in the fifteenth, but is more decidedly marked in the sixteenth, in
which vertebra the arterial canal perforates both the processes at their common base.
At the twenty-second vertebra the two processes are widely separate, but at the
twenty-third there is only one nodular process perforated at its base, and in the
twenty-fourth the groove has cut the process in two.
The oblique processes are long narrow rods in the first six vertebrae, but they
become shorter and thicker in the seventh, their characters becoming intensified in
the vertebrae succeeding it, till at last in the fourteenth they are reduced to an
obscure eminence on the body of that segment. The processes of the first caudal
embrace the base of the spinous process of the last lumbar, but in the second, third,
fourth and sixth they only reach forward to on a line with the posterior margins
of the spinous processes in front of them. All the remaining metapophyses stop
short of the spinous processes in front of them.
The spinous processes of the caudal region are all forwardly curved, their hinder
margins being convex, and their anterior margins concave, and they are of nearly
equal breadth at their bases and extremities in the first five; but from the sixth
backwards they rapidly and gradually diminish in length, their bases becoming
narrowed and their extremities expanded. Erom the eighth backwards they simulate
the form of the chevron bones, and in the ninth, tenth, eleventh, and twelfth a
small process is developed at the hinder extremity of their superior margins. The
fourteenth has the form and size of the fourteenth chevron, whereas the fifteenth
and last is only one-fourth of its size. The neural canal in the last-mentioned
vertebra is excessively small, and the process only occupies little more than one-half
of the body, which is deeply grooved behind it by the arterial canal, so that this
vertebra is intermediate in its characters between the vertebrae before it with strong
spinous processes and those behind it in which these have disappeared. In the
sixteenth vertebra, however, the remnants of the laminae are unmistakably present