134 
MR. W. K. PARKER ON THE STRUCTURE AND 
But the hyo-mandibular (h.m.) showing yet no division by two osseous centres, and 
lying behind and not within the quadrate, is quite Polypterine in character. Reference 
to my second stage (Plate II. fig. 3, h.m., c.h .), and to Dr. Traquair’s figures of Polypterus 
(fig. 6, H.M. , H.M'.), will give us an explanation of a peculiarity to be seen in that 
“Ganoid” — namely, that the top of the newly separated “ cerato-hyal” (c.h.) of the 
embryo Salmon, which is connected with the lower end by a constricted portion, has its 
counterpart completely segmented off in Polypterus; and this it is which forms the accessory 
hyo-mandibular of Traquair (op. cit. p 176); it is an “upper cerato-hyal.” The term 
“ post-hyomandibular” must not be used for this, but for the hinder condyle of the 
true “ hyo-mandibular,” a part which becomes so strangely modified in relation to the 
organ of hearing in the Frog (“ Frog’s Skull,” Plate vii. fig. 13, pp. 170, 171). The head 
of the “hyo-mandibular” of the Salmon at this stage is slightly divided into two parts; 
but although many “ Teleostei” have two condyles to this bone, in the Salmon the joint- 
cavity continues single. The “opercular knob” is at this stage a mere obtuse pro- 
jection; the cartilage narrows gently towards it, and then suddenly; and here it curls 
forwards, getting a little within the metapterygoid flap ; then, applying itself to the hind 
edge of the quadrate, it is really wedged between the quadrate and the angular process of 
the “ articular ” region. At the convexity of its bend, behind, we find a cupped space rather 
on the inside ; it is for the stylo-hyal (st.li.), and a joint-cavity here makes this a small ball- 
and-socket joint. Below this joint the cartilage belongs to the “symplectic” region; it 
becomes the nail which is driven in a slanting manner into the inner face of the quadrate. 
The “stylo-hyal”* (st.li.) does not appear until the emergence of the head from the 
egg (Fourth Stage, Plate III. figs. 1 & 6) ; it is evidently developed in the hooked apex 
of the posterior half of the primary hyoid bar in the fourth, and in the adult stage it is 
too short to show much of the curve, but now it is very evident. The apex of the anterior 
half develops the opercular process (see Plate II. figs. 3 & 6, Plate III. fig. 6, and 
Plate IV. fig. 1, op.c.). It is also to be noted that the anterior half of the bar forms the 
“ hypohyal” below, which it loses, giving it up to the next half; whilst the posterior half 
keeps its own apical segment, the “ stylo-hyal,” and gains a new distal segment, the 
“ hypohyal.” These changes of number, form, and place are to me very wonderful; yet 
I must give my account of them in patient detail, not the less that my wit will not reach 
to their meaning. The elements of this half-arch are tied to each other by fibrous bands ; 
I can find no joint-cavity between them. The stylo-hyal, true to its original character as 
an inturned hook, is articulated inside the apex of the great outflattened cerato-hyal ; 
this piece (c.h.) only gets to be two by ossification, the upper third having the epihyal 
as a centre; but the “synchondrosis” is not severed through. 
This piece is very flat above, with rounded margins ; but it contracts and thickens 
below, and then forms a swollen head, which fits into a corresponding depression in 
the distal piece, the hypohyal (h.h.) ; this latter part is very solid and is grooved below 
* These terms are purely iclitliyic, and are not to be confounded with like terms used for the higher Yerte- 
brata ; we are not quite prepared yet for an harmonious terminology. 
