While the skeleton of _Berardius bairdii_ (Pl. 42, fig. 4) resembles that
of _B. arnuxii_ very closely in most particulars, it presents differences
which may properly be regarded as specific. The vertebral formula of _B.
arnuxii_ as given by Flower is as follows: C. 7, Th. 10, L. 12, Ca. 19 =
48.[58] The same formula is given for another specimen of _B. arnuxii_ by
Van Beneden and Gervais, except that the caudals are 17, two being
apparently lacking.[59]
Doctor Hector, however, gives a different formula for a third specimen of
this species, namely, C. 7, Th. 10, L. 13, Ca. 17 = 47. He remarks that
“extreme care was taken to secure the whole of the small tail bones.”[60]
The discrepancy here shown can not be accounted for at present, but, at
all events, none of the formulas of _B. arnuxii_ corresponds to that of
_B. bairdii_, as derived from the three skeletons in the National Museum,
namely, C. 7, Th. 11, L. 12, Ca. 16+ = 46+.
The number of thoracic vertebræ can be determined positively from the
youngish male from St. George Island (Cat. No. 49727), in which ten pairs
of ribs are present, together with one rib belonging to the eleventh
pair. This last is much shorter than the tenth pair, and there can be no
doubt that it really belongs to a terminal pair. In this skeleton the
transverse processes of the eleventh thoracic vertebra are thick at the
free end like those of the tenth thoracic vertebra.
In the adult male from Centerville beach, California, only ten pairs of
ribs are present, but as the tenth is quite as long as the ninth, there
is little doubt that an eleventh pair was present originally. The
eleventh thoracic vertebra, however, has transverse processes longer and
more flattened at the free end than those of the tenth thoracic. It is
possible, of course, that the real eleventh thoracic is lacking, and that
this individual had thirteen lumbar vertebræ, but of this there is no
positive evidence.
Only a few of the ribs accompany the skeleton of the adult female from
St. George Island, Alaska (Cat. No. 49726), but there are eleven thoracic
vertebræ, the transverse processes of the eleventh being short and thick,
like those of the tenth, with a distinct facet for the rib at the free
end. This facet, however, is directed obliquely backward and occupies
only the posterior half of the free margin.
There is no doubt in my mind that the number of thoracic vertebræ in _B.
bairdii_ is normally 11 and in _B. arnuxii_, 10. This would ordinarily be
of little importance, as in nearly all kinds of cetaceans a variation of
one, or even two, in the number of thoracic and lumbar vertebræ in
different individuals of the same species is commonly met with. In the
present family, however, the number of thoracic vertebræ shows little
variation, and as all known skeletons of _B. bairdii_ have eleven
thoracics and all known skeletons of _arnuxii_ appear to have ten
thoracics, it seems probable that this difference is specific. At all
events, it is correlated with a difference in the form of the vertebræ
themselves. As is well known, the transverse processes of the thoracics
in this family undergo a sudden change of form and position near the end
of the series, the elevated processes on the anterior thoracics being
replaced on the posterior vertebræ by others at a lower level on the
sides of the centra. This change takes place differently and on different
vertebræ in the two species under consideration.