206 MR. W. K. PARKER ON THE STRUCTURE AND DEVELOPMENT 
The pharyngobranchial of the fourth arch belongs also to the fifth (fig. 3); forking 
below, it is attached to the apex of both the fourth and fifth. This latter arch has its 
epibranchial continuous with the pharyngobranchial of the fourth; it has a flat notched 
ceratobranchial piece (fig. 4, ¢.b7, 5), and has no hypobranchial. 
The sectional views further illustrate this stage of nearly ripe embryos of the Dog-fish. 
A vertically longitudinal section (Pl. XXXVI. fig. 6) shows how completely the cranial 
cavity is filled with the brain, and that the mesocephalic flexure is obliterated. The 
tegmen cranii is continued over part of the fore brain; and the floor of the cranium is 
one continuous sheet of cartilage, formed by the investing mass behind, and by ‘the 
united trabecule in front. The nasal sac (0/7) now lies in front of, as well as some- 
what beneath the fore brain. Beneath and behind the olfactory folds is seen the distal 
end of the pterygoquadrate bar (g.pg), and below the mouth the distal end of the man- 
dible (mn). The hyoid crus and base (c.hy, 6.hy) and the distal parts of the branchial 
arches are shown, as also the basihyal and basibranchial, in section (.hy, 6.br). There 
is but little of the notochord (nc) left; and a posterior clinoid ridge shows the rudi- 
ment of the “sella turcica.” The pituitary body (py) is very small; beneath it the 
internal carotid is seen entering the cranium. 
In the first of the transverse sections (Pl. XLII. fig, 5) the eyeballs are cut through, 
and a view is gained of the height and width of the cranium, built upon the foundation 
of the trabecule and their ‘“‘ commissure.” ‘The orbitosphenoidal side walls connect 
the trabecular crest with the superorbital band and the “ tegmen cranii.” Below the 
mouth (m), the oral “ labials,” the pterygoquadrate bands, the mandibles, and the fore 
part of the basihyal are cut through (¢.pg, mn, b.hy). 
In another section further backwards, and somewhat oblique (fig. 6), the hinder part 
of the eyeball (¢) and the fore part of the auditory sacs are shown, ‘The basal cartilage 
is cut through where the trabecule have coalesced with the investing mass. The third 
section (fig. 7) is through the posterior and horizontal semicircular canals (p.sc, h.sc), 
the upper part of the oblique foramen magnum (/f.m), the investing mass, and remnant 
of the notochord (iv, nc), and the head of the hyomandibular, or “ epihyal” (h.m). 
The thick auditory capsule is seen to be still distinct above from the superoccipital 
cartilage (so). The passage for the glosso-pharyngeal and vagus (9, 10) is seen below. 
The remarkable extravisceral cartilages (Pl. XXXVI. fig. 4, ex.br) are shown outside 
the branchial arches: they are sharp above, and dilated below; there are four pairs of 
them ; and the last (e.vs, 9) is very small. 
These cartilages await proper classification; at present they may be bundled up with 
the “ labials.” 
Fourth Stage. Adult Dog-fish (Scyllium canicula). 
At first sight it might be supposed that a skull without any proper ossification in 
the adult state would present the greatest difficulties to the morphologist; for the 
various elements of the cranium are here thoroughly soldered together, making a com- 
