216 MR. W. K. PARKER ON THE STRUCTURE AND DEVELOPMENT 
In an embryo of Pristivrus z!s of an inch longer than the last (? of an inch), all 
the postnasal structures of the skull-base were clean removed, and the upper wall of the 
fauces and the pharynx shown (Pl. XX XV. fig. 4). 
The parts last described are shown in situ, as in fig. 8; and the whole series of post- 
oral visceral arches are shown: they were seen as a transparent object, and the prepara- 
tion flattened a little. The pith of the facial rods took the carmine well, and had become 
true hyaline cartilage. If this figure be compared with the side and lower views of the 
younger embryo, drawn in its undissected condition (figs. 1, 2), the following description 
will be easily understood :— 
The first cleft or spiracle (c/ 1) is wide at the top and a mere chink below; its direc- 
tion during the mesocephalic bend is backwards and downwards, its opercular lip, or 
“ vallance,” looking upwards, and, from lack of breadth, exposing the fowr short external 
branchial filaments. Within this opercular fold an oval cartilage is found (fig. 4, sp.c) ; 
this is the “ spiracular cartilage,’ and corresponds to the apex of the mandibular bar of 
an embryo Salmon (“Salmon’s Skull,” pl. 1. fig. 7, mn). Compared with the Urodele 
or Batrachian, this rudiment must be considered to correspond to the “ otic process,” 
and not to the “ pedicle” (Huxley, 4, p. 42). On each side, below this spiracle, its 
operculum and its contained cartilage, the wall of the mouth (7m) is thick, and strongly 
bent upon itself; this thick part extends from the chin behind to the front of the 
mouth and the nasal sacs. Each lateral cartilage is a thick half-link. Were the two 
confluent at the mid line, they would make a transversely placed link or oval ring; but 
they lie askant, the forebent part being higher than the part behind the mouth. The 
fore part is flatter, and the hind part thicker than the rest; but they both are widened 
at the end, and turn, in a pedate manner, outwards. Behind their middle the cartilage 
is becoming somewhat loose in texture; a transverse cleft has begun in its substance ; 
and this evident break in the cell mass has its concave margin looking forwards. 
Behind this line we have the rudiment of the lower jaw, in front of it the upper; and 
the latter will have a convex end, to fit into the concavity of the former. The upper 
jaw is called the * pterygo-quadrate * bar; for the quadrate region has its apex indepen- 
dently developed, and the large foregrowth of the bar is built into the whole side of the 
palatal ceiling. The mouth, thus encircled and fringed in front by the slightly bilobate 
fronto-nasal process (fig. 2, f:7.p), is ear-shaped, a long transverse oval with a short anterior 
expansion. 
Considered as the first branchial arch, which in reality it is, we thus have an inde- 
pendently developed upper segment and a commencing division of the main bar into an 
“epibranchial” piece, which grows forwards and inwards, and a “ ceratobranchial,” 
which grows downwards and inwards. 
The next arch is similar to the last; it is the hyoid, and also has an oval cartilage 
above, and a long bar lower down, also strongly bent upon itself. 
See also the adult Skate’s skull (Pl. XLI. fig. 4, mt.pgq). 
