DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 115 
to receive the thiee convexities of the parachordal tract. The bone is gently hollow 
for the pituitary body, and this “sella turcica” is finished behind by the triple rising, 
very slight, of the postpituitary wall, the fore part of the cartilaginous tract in which 
the basisphenoid and basioccipital (b.0.) will ultimately meet. The three convexities 
of the cartilage are due to the manner in which the thick parachordal tracts met, and 
united, over the narrowing cranial end of the notochord. 
Below (fig. 1), the investing mass, or parachordal cartilage, was finished into bone 
right and left, by the basioccipital (b.0.); above, the sharp edges that lie against 
the nut-shaped cochlev (ch/.) are unfinished, the rest corresponds with what is seen 
below. 
The cochleze (ch/.) look broader in this aspect than from below; they overlap the 
out-thrust alisphenoids, somewhat ; their arched inner porch or meatus internus (VIL, 
VIII.) is seen in this view, and also the widened chinks for the 9th and 10th nerves 
(IX., X.). The bulging supraoccipital centre (s.o.) has the outer third, on each side, 
occupied with a large convex inturned crest of cartilage, which runs forwards in front 
of the hinder angle of the alisphenoid; that foremost part is the remnant of the 
great continuous band that did run into the orbitosphenoid. It is mainly the hinder 
third of the sides of the “tegmen cranii,” and is supraoccipital behind, and “ pterotic ” 
at the sides.* 
This “demonstration” will be finished when I have described the great septum of 
the nasal labyrinth in its lateral aspect (Plate 15, fig. 3). The whole of the large crest 
of the intertrabecula is rather low, its basal or primary part is very round and solid. 
A large oval fenestra (7.2.1) lies in front of the crest, surrounded by the alinasal 
growths (al.n.). 
These are figured as cut away, but from the lower part there is given off the 
recurrent or JACOBSON’s cartilage (7c.c.), a long, hollow-faced spatula, with a small 
vomerine bone (v’.) above its dilating part, in front. The aliseptal fold (al.sp.) is thin, 
but where the hinder part of the roof is cut through the aliethmoid (al.e.), there the 
cartilage is very solid. Behind it the cartilaginous crista galli (c7.g.) stands like a 
reversed “rostrum.” The margin of the septum then forms one large, nearly semi- 
circular notch along the thin end of the intertrabecular septum (p.e.). Concentrically 
with this, a little lower down, the thick, cribriform plate (c7.p.) is cut through. Behind 
the fosse, the stem of the cornu trabecule (see fig. 2, ¢.tr.) is cut through, and then 
comes the orbito-presphenoidal bone (p.s.) grooved laterally in the attempt to form 
a distinct median centre. Much cartilage is still unused, below and behind; then 
the basisphenoid (b.s.) shows itself through the thickness of the basal beam. The 
* This lateral tract is ossified separately in the Osseous Fishes as the “ pterotic” (see ‘ Salmon’s 
skull,” Phil. Trans., 1873, Plates 5-8). In the Mole and Shrew the ossification of this part (as I shall 
presently show) takes placeina remarkable manner. The opisthotic rapidly ossifies nearly all the walls 
of the labyrinth, but the prootic arises in its normal place, and then, pushed upwards, so to speak, by the 
huge opisthotic, runs into this superauditory crest, forming a ptero-prootic bone. 
Q 2 
