DEVELOPMENT OP THE SKULL IN THE MAMMALIA. 
171 
middle in this section, and also the eye-halls (■ e). Here we have apparently an open 
floor to the brain, which is the roof to the nasal labyrinth, for the cribriform plate is not 
yet chondrified. The solid septum (p>-e.) is still convex in two places, it stands quite by 
itself in the section, for the lateral ethmoidal structures at this part, are far from 
it, right and left; it is supported, below, by the vomer. There is still a small 
upper turbinal rudiment, and the middle turbinal ( m.tb .) is in two folds ; the floor 
is strong, it is still at a great distance from the mid-line. Over the eye (e.) the 
frontal (f.) is seen, and the maxillary palatine plates, those of the palatines, and the 
jugal bones ( pa ., j .) are seen in the roof and sides of the mouth. The tongue (tg.) is cut 
through its middle ; and the lower jaw is seen as a cruciform section of the dentary (cl.) 
holding a tooth-pulp (t.) above, and Meckel’s cartilage (mk .) below. 
10 th Section (Plate 23, fig. 25). —The cribriform plate (cr.p.) is here a large 
membranous tract, right and left, hollow above and convex below. 
The perpendicular ethmoid (p.e.) has lost one-third of its height, and is much 
thinner above; the floor (n.f.) has now reached it, and each plate articulates with 
its bulbous base. The wall and the last fold of the middle turbinal (m.tb.) are 
thick plates, free above, and having their concave faces looking towards each other; 
the nasal canal is thus subdivided into two passages, the outer oval, and the inner 
unciform, in section. Another cartilage has appeared above the low wall, and at a short 
distance from it; this is the orbitosphenoid (o.s .); it is thicker below than above, 
and convex outside; at a small distance outside it a much larger part of solid tissue is 
cut through, this is the frontal ( f), which, however, only forms a wall—not a roof—- 
to the huge hemisphere (CP). The vomer (v.) is still seen in section, and the palatines 
(pa.), with it, almost complete the fence round the bilobate nasopalatine passage. 
Below, the dentary (cl.) is thickening over Meckel’s cartilage (mk.). 
11 th Section (Plate 24, fig. 1). —This section is through the hind part of the nasal 
labyrinth, and the nasal passage (n.p.), right and left, is single, large, and heart-shaped. 
This is the last section through the membranous cribriform plate (cr.p.), and tire 
perpendicular ethmoid (p.e.) is but little above half its original height; the nasal wall, 
being cut through obliquely to its plane, looks extremely thick. The ascending floor 
is still articulated with the bulbous middle wall. Here the orbitosphenoid (o.s.) is 
larger, and is sharp both above and below ; it almost reaches the nasal wall. The 
frontals (f.), the palatine (p.), and the forks of the vomer (v.) are seen in section; 
also Meckel’s cartilage (mk.), and the dentary (cl.) below. 
12 th Section (Plate 24, fig. 3). —This and the next section have been accidentally 
transposed: the back wrnll of the nasal labyrinth is here seen and the fore part of 
the Gasserian ganglion (V.). 
This part shows a very deep orbitosphenoid (o.s.), resting by its thick base upon a 
cartilaginous fold growing out from the back of the nasal labyrinth ; although 
the orbitosphenoid (o.s.) was cut through, the small optic foramen was caught 
in this section, The presphenoid ( p.s .) is almost trilobate, and is composed of the 
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