In _B. arnuxii_ the eighth thoracic has no facet at the posterior end of
the centrum for the articulation of the head of a ninth rib and no
distinct transverse process, the tubercle of the rib articulating with a
facet on the side of the metapophysis. In _B. bairdii_ the eighth
thoracic is similar, but there is a distinct facet at the posterior end
of the centrum. (Pl. 32, fig. 1.)
In _B. arnuxii_ the ninth thoracic has a very distinct transverse process
on the side of the centrum, while in _B. bairdii_ the ninth thoracic has
a short, slender process attached to the side of the metapophysis and no
facet at the posterior end of the centrum. (Pl. 32, fig. 1.)
In _B. arnuxii_ the tenth thoracic is the second one having a distinct
transverse process, and the latter is broad distally and has the
articular facet on the posterior portion of the free margin. In _B.
bairdii_ the tenth thoracic is the first having a distinct transverse
process on the side of the centrum. (Pl. 32, fig. 1.)
There are only ten thoracics in _B. arnuxii_, as already mentioned, but
in _B. bairdii_ there are eleven, and the eleventh is that which bears
the second transverse process on the side of the centrum.
The foregoing differences amount to this: That in _B. bairdii_ the
commencement of the lower series of transverse processes is pushed back
one vertebra, as compared with _B. arnuxii_, and that in the ninth
thoracic of the former species, which corresponds to the eighth of the
latter species, the metapophysis has a short process on the side for the
articulation of the tubercle of the rib, instead of merely a sessile
facet. Although in other genera of ziphioids these differences would
perhaps be looked upon as individual, since they are constant here they
may be considered specific, at least provisionally.