152 
MR. W. K PARKER ON THE STRUCTURE ANO 
dosteus, and in many types, they are really continuous under the pituitary body, as 
in Scyllium and lima, forming a complete floor to the skull for some distance in front 
of the notochord and the post-clinoid wall; but this part has a small fontanelle imme¬ 
diately below the pituitary body (see fig. 1, py.). Opposite the optic nerves they part 
for a short distance, nearly meet again, and then diverge as two very short out-turned 
horns, the tips of which are the rudiments of the cornua trabeculae (< c.tr .). Between 
the Gasserian ganglion and the optic nerve (V., e.) the trabeculae grow upwards, 
forming the rudiment of the alisphenoid ( al.s .); the walls and roof, however, will be 
best studied in the sections. 
In front of the eye-balls (e.), between them and the olfactory sacs {ol.), there is a 
definite antorbital cartilage growing outwards and a little forwards from the tra¬ 
beculae ; this is the ethmo-palatine {e.pa.), a familiar element, which, here, is only 
semi-segmented from the trabecula. In Skates, as in Urodeles and Teleostei, it is 
distinct; but in Batrachia and Sharks it is not; it is well seen as a “ process ” in 
Notidanus (Heptanclius and Hexanchus, see Gegenbaur’s ‘ Selachians,’ plate 1, and 
also in embryos of Scyllium , T. Z. S., vol. 10 , plate 37, fig. 1 , a.o.). Here, it helps to 
form the back wall of the crypt in which the olfactory sac {ol.) is lodged; in these 
Fishes, as in Lepidosteus and the Teleostei, there is no distinct “ paraneural ” cartilage 
for the nasal pouch. 
Leaving the walls and roof of the skull, until I come to the sections, I shall now 
describe the visceral arches, which are greatly developed since the last stage. 
In a side view of these parts (Plate 1 3, fig. 11) I have shown the arches relative to the 
other cephalic structures, which are left in outline to give prominence to the view of 
the arches themselves. 
In another figure (Plate 13, fig. 12) the arches are shown as seen from above, and in 
a third (Plate 14, fig. 5) they are shown as seen from below. The hyoid arch is twice 
as large as the others, and has undergone most segmentation and specialization; the 
branchial arches decrease in size and in the amount of their segmentation, from 
before, backwards. 
The proper pedicle of the mandibular arch, in this extremely hyostylic type, is not 
only absent, but the stunted and forwardly projecting “pier” is much further from the 
normal point of attachment—under the outgoing fifth nerve—than in any of the 
Selachians. Each of these pterygo-quadrate bars (pg*q.) is a curved plate of cartilage, 
thick and bent forwards in its articular region, where it grows out over the temporal 
muscle, somewhat, to form the rudiment of the “ orbital- process.” and flat as it widens 
out in its pterygoid region. 
The right and left plates meet each other for some distance in front, a notch sepa¬ 
rating their round ends; they are gently convex, above, and gently concave, below. 
These bars reach as far forwards as the antorbital region, but are embedded in the 
very loosely attached palatal skin, and are thus only slightly connected with the basis 
cranii. In the angle formed by their convergence a pyriform wedge of somewhat 
