DEVELOPMENT OF THE SKULL IN STURGEONS. 
165 
symmetry, the large median piece being sub-oval and the large lateral pieces wedge- 
shaped ; but the number of the lesser segments varies, and the outermost, on the left 
side, is only half separated. In front, the edge is turned downwards (Plate 16, fig. 6), 
and here a second median cartilage appears, with three lesser segments on the right, 
and two on the left side. The whole of this “ unpaariger Gaumenknorpel ” (MUller) 
is a gently convexo-concave, two-winged structure, which finishes the hard roof of this 
remarkable tubular protractile mouth. 
The uppermost segment of the hyoid arch (Jim.) is nearly twice the height of the 
ascending, but arrested, quadrate region ; it is the largest of five “ internodes ” in this 
double-sized, forked arch, with its double function. The head of the hyomandibular is 
rounded (Plate 15, fig. 13, and Plate 16, figs. 1, 5, hm.) ; it curves backwards, and sends 
from its lower two-thirds a large thick flange, the opercular process; this causes 
the width of the bar to be more than doubled. Its lower condyle is cylindroidal, and 
the concavity on the next joint answers to it, so that it is like the humero-ulnar 
joint of a Mammal. Just the neck of the bar is ringed with an ectosteal deposit. The 
next segment, or symplectic (sy.), is scarcely half the bulk and length of the last, and 
its shape is different; it is a phalangiform cartilage, hinged to the hyomandibular, 
above, tied by ligament to the quadrate and angle of the jaw, antero-interiorly, and 
having a little concave facet inside its upper part for the inter-hyal ( iJiy .), a very 
small subquadrate segment. 
This small secondary suspensorium carries the lower part of the arch so that it lies 
inside the upper ; it is composed of two segments, for the distal fifth is segmented off; 
the main piece is the phalangiform cerato-hyal (Plate 16, figs. 1 , 4 , 5, c.hy.). The 
middle third is ossified ; the distinct piece at the end, the semi-oval hypo-hyal (h.hy.), 
is soft; they meet by their narrow rounded ends, without the intervention of a 
basi-hyal. 
The rest of the arches—the five branchials (Plate 15, fig. 13, and Plate 16, figs. 1, 4, 5) 
—are very uniform, very solid, grooved on their outer faces for the branchial vessels, and 
are quite unossified at present, and at present they have only one common basal bar, 
the basi-branchial ( b.br .), which does not reach the last arch. They form, below 
(Plate 16, fig. 4), a very regular series with the lower part of the hyoid arch ; they are, 
in their lower part, larger than the cerato-hyal at first, and then lessen, backwards. 
Only the first three have hypo-branchial segments (h.br.) ; these are larger than 
the hypo-hyal. The upper part of each arch is shorter than the lower; in the first 
four there is a single small ear-shaped pharyngo-branchial (p.br.) ; thus the last arch 
has only one piece on each side, and the last but one, three. The segmentation of the 
hyoid arch is thus seen to be very different from that of a typical branchial, which has 
no interbranchial piece, and the upper part of which is directly superimposed on the 
lower; moreover, I look upon the hyomandibular and symplectic as a divided 
epi-hyal, with no pharyngo-hyal , above. 
