100 
ME. W. IL PAEKEE ON THE STEUCTUEE AND 
fig. 1, p.mx.) is much the most massive and broad hone of the two ; it has a nasal and 
a dentary region, and its structure is peculiarly sponge-like and tubuliferous. 
The maxillary (mx.) is scooped where it is overlapped by the intermaxillary ; it sends 
inwards a scooped facet for articulation with the palatine (pa.), is rod-like in its den- 
tigerous portion, and flattens out below, especially on the upper edge, which is over- 
lapped by the malar or jugal. This third bone (Plate YI. fig. 1 ,j.) is lanceolate, scale- 
like, and fimbriated above, like the down-turned end of the maxillary ; it is only loosely 
connected with the hinge-work of the mandible. 
A very large splint covers the anterior two thirds of the cartilaginous mandible, largely 
on the outer, and less on its inner side : this is the dentary (d.). The fore end of its 
tooth-bearing part is strongly hooked ; and this hooking of the mandible, so as to fit 
into the fore end of the beak, gives a peculiar character to the Salmon’s face, especially 
in old males. On the angle of the mandible another splint is found, The angulare (ag.) ; 
it is small and rough. But the most characteristic bones investing the skull and face 
are those which form the gill-cover; these attain their highest development in the 
Teleostei. There are two sets, the “ opercular” and the “ branchiostegal for the second 
postoral arch, from which the primary opercular fold is developed, splits into a twin- - 
series of pieces at an early stage of growth. In the Ganoids, even in the Sturgeon, three 
of the four very constant opercular pieces are found on each side ; in these the “ prse- 
opercular” or proximal bone is not differentiated, and in the Polypterus (see Traquair, 
op. cit. plate vi. fig. 7, y.) the prceopercular is one with a large “temporal,” as in the 
Frog and Ostrich. This single representative of the “squamosal” and the “prseoper- 
eular” is, in the Polypterus, burrowed by mucous glands”*. 
The “ prceopercular” of the Salmon is quite subcutaneous; it has the usual falcate 
form, is burrowed in a radiating manner by mucous glands ; its attachment is by its 
fore edge, above to the hinder edge of the hyomandibular, and below to that of the 
quadrate (Plate YI. fig. 1, p.op., li.m , q.). 
Another piece developed in the proximal edge of the opercular fold is the interoper- 
cular (Plate YI. fig. 1, i.op .); it is ear-shaped, its narrow end passing within the prce- 
opercular is very thin, and it is marked concentrically and radially by growth-lines. 
The most constant of these bones in the “ Ganoids” is the “ principal opercular;” it is 
the cephalic counterpart of those scutes which lie directly below the mucous bones (see 
4 Shoulder-girdle and Sternum,’ plate i. fig. 9, op.), and the next plate behind it belongs 
to the trunk and is related to the shoulder-girdle as the “ supraclavicle.” The “ oper- 
cular” is a large subquadrate bone in the Salmon (Plate VI. fig. 1, op.) ; it is elegantly 
marked with both kinds of growth-lines, and it articulates by its own cup with a ball 
on the hyomandibular, — that process which is the morphological counterpart of the 
* I purposely mention the condition of the opercular bones in Polypterus, and that for two reasons — namely, 
to trace the Teleostean bones in each case to their simpler Ganoid representatives, and to incite Professor 
Huxley to reexamine his lettering in the woodcut in his Geological Survey Memoir (p. 22, figs. 16 & 17, 
H.M., S.T.). 
