DEVELOPMENT OE THE SKULL IN STURGEONS. 
] 63 
part of the skull is in the presphenoidal region, close behind the huge aliethmoidal and 
ethmo-palatine outgrowths ; (compare in Plate 16, figs. 1-4, ale., e.pa., tr., i.tr.). 
Here the deep middle part is formed by the hinder intercalating part of the huge 
intertrabecula (i.tr.), and the thick suborbital ledges, close beneath the optic nerves (II.), 
are expansions of the trabeculae (tr.) behind their cornua ( c.tr.). In the sectional view 
(Plate 16, fig. 2) a peninsula of cartilage is almost insulated, below, and the last two of 
the vomerine series (Plate 16, fig. 3) in front of the fore part of the parasphenoid 
( pa.s .), behind, have crept into the chinks of the cartilaginous mass. A deep, rounded 
notch, right and left, separates the antorbital from the postorbital (sphenotic) masses of 
cartilage, which are thick, round, leafy plates turning their convex margin the one to 
the other (Plate 15, fig. 13; and Plate 16, figs. 1, 3, and 4, al.e., sp.o .). 
Here, again, in front of and around the auditory capsules the cartilage has grown 
freely, yet not, at present, hiding the form of the imbedded labyrinths in the lateral 
and upper views (Plate 16, fig. 1, and Plate 15, fig. 13, a.s.c., h.s.c., p.s.c .); and here, 
in the midst of all this profusion of cartilage, there is an open “aqueduct” (aq.v.) 
between the anterior and horizontal canals A 
The trilobate fontanelle has a cranio-spinal position (Plate 15, fig. 13 ,fo), and behind 
it the cartilage runs, without division, along the spinal roof as a stout rounded process, 
whilst on each side there is a longer, arcuate, diverging process, growing from the 
epiotic and pterotic regions; the paired processes reach almost as far back as the 
branchial arches ; they end over the fourth. Even the segmentation of the skull from 
the spine is absent, as it was in the larva (Plate 14, fig. 4); and the section (Plate 16, 
fig, 2, sp.n l ~ & .) shows the exit of the first six of the spinal nerves. 
However we may interpret this continuity of the chondrocranium with the spine, it 
is a very important thing to note it fairly down. The general form of the chondro¬ 
cranium as seen from above is like that of two broadly sagittate leaves, set end to end 
by their broad leaf-stalks—the narrower orbital region; but the hinder half, as we have 
seen, is trilobate, and not simple at its free end. The arched canals (Plate 15, fig. 13, 
a.s.c., h.s.c., p.s.c.) are midway between the sphenotic lobes and the roots of the paired 
cranio-spinal processes ( c.s.p.), and the small aqueduct opens in the hollow formed by 
the arches. They project above the large lateral “eave” (Plate 16, fig. 1) ; to the 
front third of this outer projection the hyomandibular (lim.) is articulated by its convex 
condyle. Looked at laterally, the skull shows the 5th (V.) nerve emerging in front of 
the hyomandibular, the 7th (VII.) inside it, and the 9th and 10th (IX., X.) further 
back. 
The hind part of the basis cranii is strongly under-floored by the huge parasphe¬ 
noid (Plate 16, figs. 2 and 3, pa.s.), in section (Plate 16, fig. 2) it is only seen as far 
back as the 2nd spinal nerve, but beneath (Plate 16, fig. 3) its forks are shown to 
pass far back as to about the 10th; behind the parasphenoid the neuro-central 
* This passage, which I find in Siren lacertina, has been described and figured in Polyodon, by Bridge 
(Phil. Trans., 1878, Part 2, Plate 56, fig. 5, f.g., pf. } p. 699). 
Y 2 
