208 ME. "W. K. PAEKER ON THE STEUeTUEE AISTD DEVELOPMENT 



that of the pig ("appendix alee nasi"), and protects the outer edge of the lip of the 

 nasal sac (see " Pig's Skull," plate 36. fig. 1, ap. a.n). Seen from above (fig. 2) the skull 

 is a very elegant structure, with its approximated nasal roofs, its prow-like trabecular 

 rostrum {b.tr), its oval anterior remnant of the fontanelle, and its convex tegmen cranii. 

 The superorbital ridges (s.oJ), so early seen in the embryo, now give character to 

 the skull ; they are grafted upon the otic capsules behind, and upon the nasal cap- 

 sules in front, and even down in this type complicate the morphology of the ecteth- 

 moid. A superorbital foramen opens out in the front part of the groove between the 

 superorbital and the " tegmen ; " and behind the tegmen are seen the right and left 

 " aqueducts of the vestibule " {aq.v). On each side of these the elevation caused by 

 the semicircular canals is clearly seen. 



A lateral view (PI. XXXVIII. fig. 2) shows the relation of the parts very clearly, 

 and especially lights up the wing of the " antorbital," or "pars plana." Although in 

 the bird and mammal the antorbital cartilage is absent over the eye, yet it appears 

 behind in the " sphenotic " region of the otic cartilage, and in front on the lateral 

 ethmoid, running down m front of the eye as the free outstanding edge of the ect- 

 ethmoid (" Fowl's Skull," plate 81. fig. 5, p.fj, whilst the " tegmen cranii " reappears as 

 the retral spike growing from the coalesced nasal sacs (ibid, plate 83. figs. 2, 4, 5 {eth). 



Here there is no distinction of orbital and larger wings of the sphenoid ; the space 

 between the superorbital and trabecular cartilages is filled in by a continuous growth 

 of the same nature — a vertical sphenoidal wall, which is riddled with larger and smaller 

 holes, the more important of which serve as landmarks for morphological territories 

 (PI. XXXVIII. figs. 2 & 4. 2, 5, 5'). 



Besides the olfactory fenestree, there is another between the partially fused trabecular 

 cornua and nasal roofs (PI. XXXVIII. fig. 4, tr.f). The mammals do not show this ; but 

 in birds it is very common — a re-separation of the trabeculi3e from the nasal inner walls ; 

 it is not seen so low down as among the true " Struthionidce " (" Ostrich's Skull," plate 7. 

 fig. 2,8.71) — but appears in the next group, the Tinamous (ibid, plate 15, fig. 8, s.n), 

 and in the Fowl in the Tinamine stage. (" Fowl's Skull," plate 83. fig. 4, c.fc). 



The Mammalian skull is markedly prefigured by that of the Shark in the pituitary 

 region ; there is an anterior and a posterior " clinoid wall," and a floor to the sella 

 turcica perforated on each side by the internal carotid (PI. XXXVII. fig. 5, a.cl, p.cl, 

 py, i.c). This is quite unlike what is seen in Osseous Fishes, Lizards, and Birds, which 

 have no cartilaginous floor to their " sella." The notochord is replaced by cartilage ; 

 and the articulation of the skull to the " atlas " is by two condyles. 



Between the superoccipital cartilage and the proper tegmen cranii above there is a 

 crescentic fossa, looking forwards; on each side of this is the aqueductus vestibuli 

 (PI. XXXVII. fig. 2, aq.v). 



On each side of these burrowings are seen the anterior and posterior semicircular 

 canals, the latter ending in the epiotic eminence. Outside the ampulla of the 



