172 
MR. W. K. PARKER OK THE STRUCTURE AHD 
The most important further modifications to be seen in these large individuals, are 
in the visceral arches. 
There are seven arches, and the whole series is displayed in a side view (Plate 18, 
fig. 5) so as to show their relative size, and their varied modification; in that figure 
the inferior part of the hyoid arch is shown as drawn backwards towards the first 
basi-branchial (c.hy., b.hy., h.br 1 ) ; in reality it runs across to meet its fellow in the 
floor of the mouth (fig. 10, c.hy., h.hy.). 
The two first arches, the mandibular and hyoid, are intensely specialised; the other 
five are very similar to those of the Elasmobranchs, save that they have some of the 
larger bars partly ossified; and are still more like the branchial arches of Holostean 
Ganoids and Teleosteans. The upper elements of the mandibular arch, the right and 
left “ suspensoria,” are transformed into the oblique convex roof of the very mobile 
protrusible mouth, and the free mandibles are made to be antagonistic to the antero¬ 
inferior part of this complex structure. Each cartilaginous suspensorium is a broadly 
falcate plate, whose arched upper border comes in contact with that of its fellow in 
front and above; below and behind, it suddenly bends forwards and becomes very solid, 
to form the quadrate condyle (figs. 4, 5, q.c.). The whole plate is gently and sinuously 
convex above and concave below; behind, over the hinge, it thickens into a rib-like 
enlargement, and grows outwards and forwards as a superficial <e orbitar process” ( or.p .). 
The upper margin is arched regularly until near the hinge, and then is hollowed a 
little; the lower margin is twice-notched and concave. The bones applied to these 
pterygo-quadrate plates have not caused the absorption of the cartilage in this stage, 
which is relatively more extensive than in the last stage, in which the hind margin 
was notched. The outermost bones are the largest; they are the maxillaries ( mx), 
they are strongly bowed, dilated most where they meet in front of the cartilaginous 
plates, and to a lesser degree where they bind upon the orbitar processes behind. A 
large space for each adductor mandibukc ” muscle exists between the maxillaries and 
the pterygo-quadrate cartilages. Another superficial bone stands upright on the hind 
end of the maxillary; it is a high triangle, and its base is below. It binds on the thick, 
ribbed, outer edge of the suspensorium in its quadrate region ; this is the small u pre- 
opercular” (id. op), whose almost equally small counterpart exists in Lepidosteus. Along 
the lower edge of the cartilage, in front, a very jagged little bony scale is seen, overlapped 
by the fore end of the maxillary. This is the mesopterygoid (ms.pg.) ; it was not a 
separate bone in the last stage. Behind it there is a small style, with its sharp end 
behind, reaching to the end of the foremost and larger notch; this is the palatine (pa). 
Inside the plate (pg-q) there is a large parostosis, the pterygoid ; it lines all the 
cartilage except a falcate tract in front and above; this causes it to be sharply 
notched in front; this form is well shown by the shading of the cartilage (Plate 18, 
fig. 4). 
The mandible or Meckelian rod (Plate 18, figs. 5 and 8, mk.) is a very short stout 
cartilage like that of a Tadpole; the condyle is convexo-concave, and the angular 
