154 
MR, W. K. PARKER ON THE STRUCTURE AND 
Its form is like that of a phalangeal segment, it is curved backwards, gently, and has 
an enlargement there, above the middle, the rudiment of the “ opercular process.” Its 
axis is coincident with that of the symplectic and the mandible ( sy.,mn ., mk.), together 
they form a crescentic series, passing downwards and gently forwards, and having the 
convex margin behind ; they are bent inwards below (fig. 12). The curve forward, as 
well as downward, brings the lower end of the symplectic under the pituitary body; this 
cartilage (sy.) is only one-fourtli the length of the upper piece (hm.), but it is as thick 
as its thickest part; it is scooped above for the hyomandibular, and rounded below, 
where it is tied, by ligaments, to the two elements of the first arch (pg.q., mn., mk.). 
In the Teleostei the inter-hyal is articulated to the inside of the non-segment ed 
cartilage uniting the hyo-mandibular and the symplectic ; in the Sturgeon it is 
articulated to the inside of the distinct symplectic, rather below the middle—at least 
at this stage (Plate 13, fig. 11, i.hy.). The inter-hyal is a small piece of cartilage, 
wedge-shaped, with its narrow end upwards ; its broad lower end articulates with the 
top of the cerato-hyal, which is bilobate above, the inter-hyal articulating with the 
larger upper lobe (fig. 12, i.hy., c.hy.). 
The cerato-hyal is a thickish, somewhat- sigmoid bar, it passes inwards and forwards 
and nearly meets its fellow of the opposite side ; there is no basi-hyal, but a semi¬ 
oval segment is formed out of the ventral end of each cerato-hyal; this is the 
hypo-hyal ( h.hy .). 
The branchial arches (Plate 13, figs. 11, 12, and Plate 14, fig. 5) are larger than the 
lower part of the hyoid at first, but they lessen backwards, so that the last is only 
one-fourth as solid as the first. The first three develop a hypo-branchial segment 
{li.br.) exactly like the hypo-hyal, but larger. The inturned dorsal end of all but the 
last is segmented off as an epi-branchial ( e.br .), and its apex becomes distinct as a 
small pharyngo-branchial ( p.br.) in the first four arches. 
The basi-branchial (b.br.) is a thickish rod, rounded in front and compressed behind; 
it only carries the first three arches ; the fourth and fifth meet below. 
Bony scutes are now developing in the skin, and in the skin of the mouth and the 
mucous membrane of the palate and fauces several bony plates have appeared; all 
these are determinable. 
In the upper and lower views (Plate 13, fig. 12, and Plate 14, fig. 5) these plates, 
most of which are dentigerous, are shown in their relation to the visceral arches and the 
largest of these bones ; the maxillary and dentary—of one side—are shown separately 
(Plate 14, fig. 6, mx., d.). There is no determinable premaxillary, which like the nasals, 
and vomers, only exists as one or many of a great number of generalised scutes that are 
to be found in those respective regions. The maxillary (fig. 5, mx.) runs across and 
meets its fellow in front of the pterygo-quadrate (j>gxj.) ; it is a long, subarcuate bar, 
with thickened edges, and carrying five or six sharp, recurved teeth on its anterior 
third ; it is somewhat notched and bent in its hinder third. The dentary (d.) is almost 
