OF THE SKULL IN SHARKS AND SKATES. 219 
these are marks of the primary pituitary space (py), the soft tract between the bowed 
edges of the trabeculae. The trabecule have developed a common basal piece, the 
“basitrabecular ” rostrum (d.¢r); but this is not segmented off as a distinct bar. Where 
the trabecule have so completely coalesced in front, forming also their basal rostrum, 
they turn upwards into the frontal wall of the face, and finish the cranial floor. ‘The 
nasal sacs are hollow inverted cups of cartilage, with their downturned mouth stopped 
largely by “Jlabials.” The rim of the cup is strong on the outside, and also gives 
attachment to an ethmo-palatine cartilage (a.0). 
The roof, also, of the nasal sac is modified in its form by reason of the engrafting upon 
it of a large bowed cartilage, the “ superorbital ” (fig. 4, s.ob), a cartilage which has but 
little independence of growth, but the substance of which early appears in the embryo. 
The lower edge of this cartilage, mesiad of the eye-ball, is continuous with the trabe- 
cular crest; and the “ tegmen cranii” grows directly from it towards the mid line of the 
roof. The roof and side walls are analogous to the upper portion of a vertebral arch. 
Here, however, in the Skate, the tegmen is largely undeveloped; the brain-sac is 
permanently membranous above in front, and rests upon the laminar trabecule. Then, 
just in front of the nasal region, there is a cartilaginous beam thrown over; but 
it is narrow, and thence to the ear-sacs the roof is bare of cartilage. Behind, the 
tegmen reappears, and helps the superoccipital to roof in the hinder brain between the 
auditory masses (aw). The superorbital are grafts itself also on these sacs; hence the 
compound region, which ossifies separately in the “ Teleostei,” behind the orbit, the 
so-called postfrontal or sphenotic. 
The clear but cheese-like cartilage shows the three canals through its walls (a.s.c, 
h.s.c, p.s.c); and where the anterior and posterior of these unite, the “ aqueeductus vesti- 
buli” is seen open. 
Behind, a mass of notochord is still to be seen, and the parachordal cartilages project 
backwards outside, to form the occipital condyles (0c.c). The epiotic elevation over the 
- junction of the anterior and posterior canals is slight; the pterotic ridge outside the 
horizontal canal is well developed. 
The oral and pharyngeal visceral arches (the first or trabecular has been described as 
part of the chondrocranium) are nowhere more instructively developed than in the Rays. 
Keeping the eye upon the early condition of these parts (Pls. XX XV. and XXXIX.), 
we shall see what metamorphic results have been brought about. 
The apex of the first postoral or mandibular arch has been developed as a distinct 
crescentic cartilage, the “ spiracular cartilage” or metapterygoid (Pl. XL. fig. 4, mt.pq) ; 
it is attached below the sphenotic process, and behind the fifth nerve; it is the bearer 
of the “ pseudo-branchia ” (ps.br'), lies in the anterior wall of the first cleft, and answers 
to the “otic process” of an Amphibian. This detached suspensorium is joined to the 
quadrate region by a ligament; it answers to the hinder fork of the visceral rod. The 
larger anterior fork, the posterior extremity of which is part of the main descending bar 
