The first seven pairs of ribs have both tubercle and head. The first is
nearly as long as the second, and is very broad at the proximal end. In
the seventh pair the head is double, one facet of the rib articulating
with the facet on the posterior margin of the centrum of the sixth
thoracic vertebra and the other with the short transverse process on the
side of the centrum of the seventh thoracic vertebra. The eighth and
ninth pairs of ribs articulate only with the transverse processes of the
eighth and ninth thoracic vertebræ, respectively. The ninth pair of ribs,
as already stated, is nearly or quite as long as the eighth, from which
it seems probable that a tenth short pair was present originally. There
is, however, no trace of a facet for the articulation of such a rib on
the end of the transverse process of what appears to be the first lumbar
vertebra.
The only difference between the ribs of _M. europæus_ and those of _M.
bidens_ appears to be that the first pair is much longer proportionately
in the former species.