OF THE SKULL IN LEPIDOSTEUS OSSEUS. 
481 
This rugous bone is broadest behind, where it is scooped for the first 'vertebra; above, 
it is flattened; its fore end is emarginate, and its two oblique antero-lateral faces are 
joined by suture to the exoccipitals (e.o.) 
These bones are small lunate tracts behind the passages for the 9th and 10th nerves 
(IX., X.); the rest of the occipital arch is devoid of bone, for the super occipital is 
absent, as in the Amphibia; that region projects, as an obtuse angle of cartilage, over 
the foramen magnum. 
The auditory capsules project well into the basal plate (iv.); their canals ( a.s.c ., 
h.s.c., p.s.c.) are large, and easily seen through the transparent cartilage ; both above 
and below they send their diverticula inwards towards the mid line, so as to make the 
roof and floor of the skull into the shape of an hour-glass. The upper part spreads 
outwards over the horizontal canal, covering the facet for the hyomandibular ( hm.c.) ; 
further inwards, below, the swelling “sacculus” on each side makes a notable bulging, 
which is partly floored by the parasphenoid (pa.s.). Postero-laterally, the capsules 
are but slightly angulated ; but in front they grow outwards and forwards into an 
ear-shaped projection, which is separated by a round notch from the root of the super¬ 
orbital band (. s.ob.c.). That process is the “ sphenotic” outgrowth of the chondrocranium 
in front of the capsule; it is becoming bony (Plate 38, figs. I, 2, sp.o.); also below, in 
front of the capsule, and surrounding the chinks and openings for the 5th and 7th 
nerves (V., VII.), the prootic centre ( pr.o .) is spreading into the cartilage ; in front it 
runs into the back of the corresponding basipterygoid (Plate 38, fig. 3, b.pg.) ; and 
behind it has reached the concavity for the sacculus (vb.) 
Also above (figs. 1 and 4), an irregular bony tract is seen imperfectly divided into 
two patches, which lie over the ampulla of the posterior and the end of the horizontal 
canal (p.s.c., h.s.c.). The upper part is the rudiment of the epiotic (ep.), the lower 
portion of the opisthotic (op.). 
In front of the basipterygoid (figs. 2, 3, b.pg.), and rising upwards from it into the 
limited tract of cartilaginous wall between the orbito-sphenoidal fenestra and the ear- 
capsule, there is a bony tract, smaller than the prootic and next in front of it; this is 
the alisphenoid (al.s.). 
Along the skull base, in front of the projecting basioccipital, there is no intrinsic 
bony centre, and laterally, the whole orbito-sphenoidal region is membranous. 
But where the skull is closing in, in front, the rapidly narrowing cranium has a short 
tract of cartilage in its sides ; this is the lateral ethmoidal region ; the free border of 
this cartilage in front of the fenestra is ossified as a crescentic patch (Plate 38, figs. 2, 
3, o.s.f., l.eth.) ; this answers to the so-called prefrontal of the Teleostei, but it does 
not grow out into ethmoidal wings, as in those types. 
A very important change has taken place inside the basis cranii, for now there is a 
very definite “ posterior clinoid” bridge, not wall, of cartilage (Plate 38, fig. 3 , p.cl.) ; it 
is small, very narrow in the middle, and runs straight across, joining the roots of the 
trabeculae (tr.) together, but lying only at a small height above them. 
3 Q 2 
