_External dermal bones associated with the olfactory region._--The association of the premaxillary (_pmax._) to the nasal cartilages is described in preceding sections. The premaxillaries are separated from each other medially and from the maxillaries laterally by dense connective tissue. Anteriorly, the maxillary (_max._) bears a small palatine process (_pal. proc._, Fig. 1) and a long, delicate pars facialis (_p. fac._, Fig. 6), which terminates dorsally at the level of the lamina inferior. Posterior to the transition zone between the planum antorbitale and solum nasi the pars facialis is greatly reduced. The pars palatina (_p. pal._, Fig. 6) persists to the posterior part of the orbit. The anterior end of the prevomer (_pvom._, Fig. 1) is associated with the venter of the solum nasi at the level of the infundibulum just posterior to the incorporation of the inferior prenasal cartilage into the solum (Fig. 7). The prevomer expands dorsally around the distal end of the solum to provide a bony lateral support for the olfactory eminence (Figs. 10 and 11). A distal wing of the prevomer forms the bony anterior and medial margins of the internal nares. The palatine (_pal._, Figs. 1 and 12) lies in connective tissue medial and adjacent to the pars facialis. At its maximum size the palatine forms the bony posterior margin of the internal nares and extends dorsomedially from the pars palatina to the distal part of the solum nasi. The nasal (_nas._, Fig. 1) is a thin bone overlying the tectum nasi anteriorly (Fig. 7). It expands laterally to form a complete roof over the cavum principale (Fig. 10). In the region of the internal nares, the nasal forms the lateral wall of the cavum principale (Fig. 12). Sphenethmoid Region Posterior to the transition zone between the planum antorbitale and solum nasi, the sphenethmoid (_spheth._, Fig. 1) is fully ossified medially, the lateral parts of the bone at this level are only ossified perichondrally. The septum nasi persists at the anterior level of the orbit and terminates just anterior to the orbitonasal foramen (_orbnas. f._) and the anterior end of the parasphenoid (_pasph._, Fig. 13). The orbitonasal foramen is moderately large, has a complete bony margin, and is located at the dorsolateral corner of the braincase. [Illustration: FIG. 13. Transverse section through sphenethmoid region at level of orbitonasal foramen. Abbreviations: _ant. max. proc._, anterior maxillary process; _max._, maxillary; _orbnas. f._, orbitonasal foramen; _pasph._, parasphenoid; _spheth._, sphenethmoid.] At the level of the orbitonasal foramen, the sphenethmoid is entirely ossified except for a small dorsolateral extension. This distal extension expands laterally in posterior sections as the braincase is increased to its maximum width at the mid-length of the orbit; the cartilaginous margin is retained throughout the length of the sphenethmoid.