4. _Membrane-bones of the alimentary portion of the visceral skeleton of the skull_.--The suspensorium has one tegumentary bone attached to it, viz. the _præoperculum_ (30); it is but rarely absent, for instance in _Murænophis_. The _premaxillary_ (17) and _maxillary_ (18) of the Teleostei appear to be also membrane-bones, although they are clearly analogous to the upper labial cartilages of the Sharks. The premaxillaries sometimes coalesce into a single piece (as in _Diodon_, _Mormyrus_), or they are firmly united with the maxillaries (as in all _Gymnodonts_, _Serrasalmo_, etc.) The relative position and connection of these two bones differs much, and is a valuable character in the discrimination of the various families. In some, the front margin of the jaw is formed by the premaxillary only, the two bones having a parallel position, as it has been described in the Perch (p. 53); in others, the premaxillary is shortened, allowing the maxillary to enter, and to complete, the margin of the upper jaw; and finally, in many no part of the maxillary is situated behind the premaxillary, but the entire bone is attached to the end of the premaxillary, forming its continuation. In the last case the maxillary may be quite abortive. The mobility of the upper jaw is greatest in those fishes in which the premaxillary alone forms its margin. The form of the premaxillary is subject to great variation: the beak of _Belone_, _Xiphias_ is formed by the prolonged and coalesced premaxillaries. The maxillary consists sometimes of one piece, sometimes of two or three. The principal membrane-bone of the mandible is the _dentary_ (34), to which is added the _angular_ (36) and rarely a smaller one, the _splenial_ or os _operculare_, which is situated at the inside of the articulary. 5. _Cartilage-bones of the respiratory portion of the visceral skeleton of the skull._--With few exceptions all the ossifications of the hyoid and branchial arches, as described above (p. 58), belong to this group. 6. _Membrane-bones of the respiratory portion of the visceral skeleton of the skull._--They are the following: the opercular pieces, viz. _operculum_ (28), _sub-operculum_ (32), and _interoperculum_ (33). The last of these is the least constant; it may be entirely absent, and represented by a ligament extending from the mandible to the hyoid. The _urohyal_ (42) which separates the musculi sternohyoidei, and serves for an increased surface of their insertion; and finally the _branchiostegals_ (43), which vary greatly in number, but are always fixed to the cerato- and epi-hyals.