erygoid (tympanal, Cuvier) are bones peculiar
to fish. In developing Amphibia Huxley found a suspensorium of hyoid and
mandibular arches similar to the hyomandibular of fish.

Tackling his main problem of the unity of plan of the vertebrate skull,
Huxley shows, by a careful discussion of the anatomical relationships of
the chief bones in typical examples of all vertebrate classes, that
there is on the whole unity of plan as regards the osseous skull. This
unity of composition can be established, on the gradation method, by
considering the connections of the bones of the skull with one another,
their relations to the parts of the brain and to the foramina of the
principal cranial nerves. The assistance of the embryological method is,
however, necessary in determining many points with regard to the bones
developed in relation to the visceral arches. But there is a further
step to be taken. "Admitting ... that a general unity of plan pervades
the organisation of the ossified skull, the important fact remains that
many vertebrated animals--all those fishes, in fact, which are known as
_Elasmobranchii_, _Marsipobranchii_, _Pharyngobranchii_ and _Dipnoi_
have no bony skull at all, at least in the sense in which the words have
hitherto been used" (p. 571). The membranous or cartilaginous skull of
these fishes shows a general resemblance in its main features to the
ossified skull of other Vertebrates; the relations of the ear to the
vagus and trigeminal nerves are, for instance, the same in both; the
main regions of the cartilaginous skull can be homologised with definite
bones or groups of bones in the bony skull; but discrepancies occur. It
is again to development that we must turn to discover the true
relationship of the cartilaginous to the ossified skull. "The study of
the development of the ossified vertebrate skull ... satisfactorily
proves that the adult crania of the lower _Vertebrata_ are but special
developments[220] of conditions through which the embryonic crania of
the highest members of the sub-kingdom pass" (p. 573). It is with the
embryonic cranium of higher Vertebrates that the adult skull of the
lower fishes must be compared, and the comparison will show a
substantial though not a complete agreement between them. Thus, speaking
of the development of the frog's skull, Huxley writes:--"If, bearing in
mind the changes which are undergone by the palatosuspensorial
apparatus, ... we now compare the stages of develo