218 MR. W. K. PARKER ON THE STRUCTURE AND DEVELOPMENT 
the flattened eye-balls are carefully lodged in sockets, the inner half of which is formed 
by the semicircular superorbital cartilages, the horns of which are grafted on to the 
nasal sac in front and to the auditory sac behind. Outside the posterior horn of the 
superorbital (sphenotic process) is the first postoral cleft, or spiracle (Pl. XL. figs. 2 & 3, 
cl 1); its narrow inferior part bas been filled in, and only the upper expanded end has 
kept open—widely open, however, and showing on the spiracular or metapterygoid car- 
tilage a comb-like “‘ pseudo-branchia ” (ps.d7), composed of about eight branchial papille. 
Behind the spiracle the gill-bearing hyoid and the proper branchial arches are seen 
elegantly spreading into a U-shaped system of pouches, growing, as it were from a mas- 
sive stalk, the occiput and spine. 
Below, behind the basitrabecular beak, the fronto-nasal process (n.f.p) is very large 
and persistent, forming a free, anterior, emarginate lip, with the valvular nasal openings 
(na) in their primordial ventral position, with enclosed “ labials” to guard the openings. 
The angles of the mouth are lipped at right angles to the transverse inferior oral 
opening; but the mandible has its gums and teeth quite bare. The angulo-labial fold 
is continued backwards as part of the general opercular skin, which further backwards 
and outwards is imperfect in five places on each side. ‘These places are the branchial 
clefts; they are the retained lower ends of the huge primary clefts. The general oper- 
cular fold which has covered the open grating to so large an extent is seen to develop 
a special ear-shaped flap to each of the branchial outlets. 
Embryos as large as this, when carefully examined, have protruding from their 
branchial clefts what might, at first sight, be mistaken for a parasitic growth of fila- 
mentous conferve. These, however, are the still retained external branchie. They have 
begun to shorten in the first opening; but most of them are very long (Pl. XL. fig. 1, 
e.br). They are spatulate at their free ends; and their single vascular loop is still 
functional. 
Between and behind the branchial apparatus is the large, short umbilicus (w), which 
connects the embryo with a yolk the size of a dove’s egg. 
The chondrocranium, with its appended basketwork of visceral arches, is now com- 
plete, both as to chondrification and segmentation. 
On the whole very similar to that of the Shark at the same stage (Pl. XXXVI. 
figs. 3, 4, 5, and Pl. XL. figs. 4, 5, 6), it yet differs in several important points. 
The cranium seen from below (Pl. XL. fig. 6), with the eye-balls removed, is seen to 
be composed of four pairs of primary elements. Behind, the “ parachordal”’ cartilages 
that invest the notochord (¢ v, nc) have coalesced with the hinder pair of ‘ paraneural” 
capsules, those of the ear. This four-sided shorter hind part of the basis cranii has 
coalesced in front with another pair of outspread cartilages—the trabecule (tr); and 
these, also, in front have coalesced with the foremost paraneurals—those of the nose 
(na). Where the four basal plates meet, the internal carotid arteries (7.c) enter; and 
in front of these is the appearance of a space and a slit, not so densely chondrified : 
