DEVELOPMENT OF THE SKULL TjST THE MAMMALIA. 
11 
The rest of the base of the skull forms a low triangle whose base is somewhat longer 
than the lower part which reaches backwards to the foramen magnum. 
The postero-superior side of this triangle is developed into two rounded lobes, the 
upper or fore lobe is the larger of the two. The hinder lobe is the top of the 
perpendicular ethmoid (p.e.), behind the olfactory fossa with its cribriform plate (see 
Plate 5, fig. 1) ; whilst the front or upper enlargement is the crista galli, in solid 
cartilage. * 
The longer upper side dips down gently to the snout, and is a little concave ; the 
triangle does not end in a point, but in a rounded lobe. The lower part of all this 
nasal fore half of the basis cranii is a thick beam of cartilage, carrying a much thinner 
crest on its upper edge, which is continuous with it. 
Embryologically, the “ investing mass” may be said to reach to the posterior clinoid 
wall; but the notochord has retreated relatively from that point; the rest of the 
cartilage is “ trabecular.” 
But the paired trabeculae, themselves, only reach as far as to the middle of the 
proper olfactory region, the septum of which bears the name of “ perpendicular 
ethmoid ” (p.e.) ; the filling in between these, behind, and all the rest of the middle 
part is formed by an azygous growth, the “intertrabecula.” 
The sides of the cartilaginous basin, whose narrow elongated floor has just been 
described, are formed by a very large sheet of cartilage, right and left. This just 
reaches the roof, or cover of the basin, in the hinder half, and the sides are imperfect 
in two places. 
The occipital region, only, forms a complete belt or ring of cartilage. 
The postpituitary region of the skull, for two-thirds of its horizontal, and for two- 
thirds of its vertical, extent, is occupied by the intruded auditory capsules (a.s.c., chi.), 
which, above, below, and behind, have coalesced with the cartilage of the proper 
chondrocranium. 
A more than semicircular space in front of these large capsules is occupied by a 
large fenestra whose very accurate arched outline looks forwards and upwards. The 
sinuous cartilaginous wall round this space is the orbitosphenoidal region, passing by 
a band, equal in width to the fenestra above, into the large supratemporal (or suh- 
parietal) tract ( s.t.c .), which in turn runs into the crown of the occipital arch. 
Half the preauditory fenestra is imperfectly occluded by the alisphenoid (above b.s.), 
* In my paper on the Ostrich’s skull ( Phil. Trans., 1866, Plate 7) the counterpart of this crest is 
lettered cr.g., or crista galli, and is described as such in the text (p. 118). But in the paper on the 
Fowl’s skull (Phil. Trans., 1869, Plate 83) this part is not lettered, for I had been reminded by a friendly 
critic that the crista galli of Man is formed by ossification of fibrous tissue in the fore part of the falx 
cerebri. It is, however, preformed in cartilage in many Mammals, notably in these Edentata, and my 
lettering in the figures of the Ostrich’s skull is correct. In the same paper (p. 120) the quadrate 
cartilage is identified with the incus ; that view of it is, I feel satisfied, now, quite correct. 
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