THE SKELETON OF THE VERTEBRATE HEAD. 153 



be attributed to incomplete mesial fusion of a pair of " pre- 

 frontals," but on the remarkable prolongation backwards on 

 eacli of its sides of the neurapophyseal walls of the ethmoidal 

 olfactory chambers. 



Professor Owen, in his series of graphic and valuable 

 memoirs on these three extinct forms, and in his memoirs on 

 Apteryx, assuming the pre-frontal doctrine regarding the bone 

 in question, and directing special attention to the more or less 

 complete passage backwards of the nasal chambers to the an- 

 terior or inferior wall of the cranial cavity, and to the passage 

 of the olfactory nerves into these by a number of orifices, 

 apparently recognises in Apteryx, for instance (although he 

 does not directly make the statement), a completed mammalian 

 ethmoid. Now, recalling attention again to the embryological 

 considerations from which the formation of neither the 

 mammalian ethmoidal septum, nor the so-called ethmoid of 

 the Ostrich, Dinornis, Dodo, nor Apteryx, can be conceived as 

 resulting from coalesced pre-frontals, I would remark, that 

 the arrangement of the nasal fossae in Apteryx, instead of 

 being mammalian, presents the peculiar ornithic character of 

 its parts, fully brought out ; all the phases in the develop- 

 ment of which may be observed in the series of birds. In all 

 birds, the posterior extremities of the cartilaginous pouchlike 

 ethmoido-neural, or olfactory chambers, approach or encroach 

 upon the sides of the pre-sphenoidal centrum ; so that the 

 membrane, which connects its anterior margin to the cartila- 

 ginous nasal septum, and a certain extent of both its surfaces, 

 separates the two pouches from one another. The laminar or 

 hypopophyseal process on each side of the bone, variously 

 modified in form, limits, posteriorly and inferiorly, the olfactory 

 portion of the lateral surface of the bone, and, folded over the 

 pouch, walls it in more or less from below ; while the 

 lachrymal from above passes down on its outer side. The 

 gradual environment of the pouch may be traced in the series 



