DEVELOPMENT OF THE SKULL IN THE SALMON. 
109 
Salmon’s skull ; I am, of course, supposing an equivalent growth from the “ superocci- 
pital.” When we come to the earlier stages of these parts the meaning of what is here 
written will become much plainer, and the reader will do well to keep the adult con- 
dition of the various parts always before him whilst studying their incipient stages. 
With regard to the next arch, the “ pterygo-palatine,” I must confess that the state 
of things in the Frog made me hesitate in classing this arch with the rest as a morpho- 
logical equal. It certainly has a sluggish development even in the Osseous Fish, where 
it attains to its fullest growth and is for a time entirely distinct; moreover it only 
acquires the typical sigmoid form of the facial arches just as it is losing its distinctness 
from the first postoral bar. This arch is but little developed in the Urodeles, and 
scarcely chondrifies at all in the Sauropsida and Mammalia. Yet my figure of it in the 
Chick (“ Fowl’s Skull,” Plate lxxxi. pa., pg.) is perfectly correct, and my early stages 
of the Salmon will show a marvellous amount of harmony between the “ Teleostean ” 
Fish and the “ Carinate ” Bird in this and in many other parts. 
To put the matter clear at once with regard to the Frog, let me repeat what I have 
described in that creature — namely, that at first there is no pterygo-palatine arch what- 
ever ; that it is fairly suppressed during the whole of the proper ichthyic or larval period, 
only existing as a feeble “ secondary connective;” that when the tail is disappearing and 
the lungs developing it only has reached to the condition of the Lamprey ; and, finally, 
that it never becomes distinct, either from the trabecula in front or from the mandibular 
pier behind. I am somewhat inclined to attribute the feeble and slow development of 
the second preoral arch in the Salmon to the prepotent growth of the eyeball ; yet that 
will not account for its suppression in the Frog, where the eyeball is relatively much 
smaller (see “Frog’s Skull,” Plate iv. fig. 1, and Plate v. fig. 1). 
When the investing parosteal bones are removed from the facial arches of the Salmon 
we have what is displayed in Plate YI. fig. 2, and Plate VIII. fig. 9. Let an imaginary 
line be drawn through the substance of the cartilage that separates the “ mesopterygoid ” 
and pterygoid (m.pg., pg.) in front from the metapterygoid and quadrate ( [mtpg ., q.) 
behind : such a line will pass through a knob of cartilage above ; this is where the 
“ orbitar ” process of the mandibular arch has coalesced with the inturned or hooked 
upper ( = posterior) end of the pterygo-palatine rod, and the line drawn will show the 
whole extent of the coalescence. These parts do not coalesce in the “Abranchiate 
Vertebrata.” The dentigerous bony palatine does not wholly ossify the palatal region 
of the bar, a large knob articulates with the maxillary near the end of the prepalatal 
spur ; the “ ethmo-palatal,” an earlier process, tied to the trabecular facet by strong fibres, 
does not ossify, and the postpalatal portion has a soft core. The “mesopterygoid” 
(m.pg.) overhangs the outer side (Plate VI. fig. 2), but is large, convex below, inturned, 
then concave, and then flat behind, on the inner side (Plate VIII. fig. 9). This fiat 
posterior portion overlaps the pterygoid ( pg.), which clamps the lower edge of the bar 
behind, is most developed on the inner side, and, like the “ mesopterygoid,” overlaps 
the inner face of the quadrate bone. Plere there is no “ os transversum ” or “ ecto- 
