The vertebral formula of three specimens of _M. bidens_ and of the
Atlantic City specimen of _M. europæus_ is as follows:
M. europæus.
Atlantic City C. 7; Th. 9; L. 11; Ca. 20=47
M. bidens.
Landenæs 7; 10; 11; 19=47
Fæø 7; 9; 11; 19=46
Udsire 7; 10; 9; 20=46
Although the skeleton of _M. europæus_ appears from the foregoing formula
to include one less thoracic vertebra than those of _M. bidens_, as the
last pair of ribs present is as long as the preceding ones, an additional
pair probably existed originally. The formula for _europæus_ would then
be: C. 7, Th. 10, L. 10, Ca. 20 = 47. (Pl. 13, fig. 1.)
In the Atlantic City specimen all the epiphyses are free. The atlas and
axis are anchylosed together, the third cervical is united to the axis by
the centrum, and on the right side by the top of the neural arch; on the
left side the arch is imperfect and free. The fourth to the seventh
cervicals, inclusive, are all free. The arch is incomplete above in the
fourth, fifth, and sixth, but complete in the seventh. There is a short
neural spine on both sixth and seventh cervicals. The atlas has a broad,
obliquely-truncated inferior lateral process, but no superior process,
while the axis has both inferior and superior processes. The inferior
process is twice as long as the superior process, and both are directed
backward. They do not meet to form a ring. The third to the sixth
cervicals, inclusive, have inferior processes only, that on the third
being long and thin (but developed on the left side only). On the fourth
and fifth cervicals the processes are short and small; on the sixth, long
and broad, and directed downward. The centrum of the seventh cervical has
a broad facet on the side, where the first rib is attached, and an
inferior lateral process thicker than that of the sixth cervical, but
also directed downward.
It is doubtful whether the foregoing characters of the cervical vertebræ
are of any systematic importance, as there is a very large amount of
individual variation among these animals in the development of the
transverse processes and other details of structure. _M. bidens_,
however, appears to have superior transverse processes on most of the
cervicals which sometimes unite with the inferior processes to form
foramina. In the specimen of _M. europæus_ under consideration there are
no superior processes, except on the axis.
Metapophyses are first distinguishable on the diapophyses of the fourth
thoracic vertebra, and on the seventh assume the form of conical
tubercles. On the eighth and following vertebræ they are flat, and are
last distinguishable on the seventh caudal vertebra. Facets for the
articulation of the tubercles of the ribs occur on the diapophyses of the
first to the seventh thoracic vertebræ. On the latter vertebra the first
transverse process appears as a short projection on the side of the
centrum. On the eighth thoracic vertebra, the transverse process is broad
and flat, with the anterior margin bent upward, and is about 48 mm. long.
The base of the neural arch is strongly concave externally. The
transverse process of the ninth thoracic vertebra is similar to the
preceding one, but broader and not bent upward anteriorly. The base of
the neural arch is also concave in this vertebra. The ends of the
transverse processes of the eighth and ninth vertebræ are emarginate for
the articulation of the ribs. A median inferior ridge is first
distinguishable on the seventh thoracic vertebra.
As far as can be learned from the descriptions of Turner, Grieg, and
others, the thoracic vertebræ of _europæus_ do not present any marked
differences from those of _bidens_.
The transverse processes of the lumbar vertebræ are short, broad, and
flat, and somewhat curved forward. They are expanded and rounded at the
free ends. The centra increase in length posteriorly, the last lumbar
having the greatest length of any vertebra in the column. The neural
spines increase in length from the first lumbar to the fourth, those on
the remaining lumbars being subequal, but the spine on the ninth lumbar
is a little longer than the others. Median inferior ridges occur on all
the lumbars and are strongest at the middle of the series. The height of
the centrum of the ninth lumbar is 63 mm., width 73, and length 116. The
highest neural spine is 233.
As above mentioned, the first of the vertebræ counted among the lumbars
may be the last thoracic vertebra, but as there is no indication of an
articular facet at the end of the transverse process it is not so
considered in this place.
The lumbar vertebræ in _M. bidens_ appears to be more nearly equal in
length than in the present species, but are not different otherwise.
The spines of the caudal vertebræ decrease rapidly in height posteriorly,
and disappear after the tenth caudal. The transverse processes resemble
those of the lumbars, but are shorter. They are last distinguishable on
the eighth caudal. The transverse process of the seventh caudal is
perforated by a vertical foramen. Similar but much smaller foramina occur
on the sides of the centra of the eighth and ninth caudals. In these
vertebræ the inferior ridges are also pierced by foramina. In the fourth
caudal a ridge appears on the side of the neural arch on a level with the
top of the centrum, and similar ridges are found on the succeeding
vertebræ as far as the ninth caudal. The last ten vertebræ are without
processes or neural arches.
Sir William Turner states that the caudals of _M. bidens_ are without
vertical foramina, but the figure in Van Beneden and Gervais’ Osteography
(plate 22) shows them in the same position as in _M. europæus_. The
inferior ridges, however, appear to be imperforate in the former species.