DEVELOPMENT OF THE SKULL IN THE SALMON. 
101 
“ extrastapedial” in the Anurous Amphibia ancl in the Sauropsida. The bone lying 
below and somewhat within the opercular is the “ subopercular” (Plate YI. fig. 1, 
s.op.); it is subfalcate, broad in front, very thin, and is elegantly marked by growth- 
lines. Together these four bones form the framework to the great outer and upper 
gill-valve in the osseous fishes ; they are there subcutaneous bones, although they are 
represented by true cartilages in the “ Plagiostomes and the principal opercular piece 
is also represented by a cartilaginous sickle in the young Frog; it becomes, as I have 
already shown, the Batrachian “annulus tympanicus” (“Skull of Frog,” Plate vm. 
fig. 7, a.t.). 
Along the infero-posterior division of the second postoral arch there are twelve rays 
on each side (Plate VI. fig. 1, br.s.); these are flat, thin, and shaped like a knife-blade; 
their attachment is to the lower margin of the “ epiceratohyal” bar on its outer side. 
These rays decrease in size from above downwards ; they form one continuous series : in 
Teleostei generally, especially the Acanthopteri and Anacanthini, there are seven on 
each side ; these are terete rods, arranged in two groups — the upper four attached to the 
outside ; and the three lower end to the inside of the hyoid cornu. Plere, again, the 
-Salmon is seen to be only subtypical. There is an azygous bone at the base of this 
series (Plate VI. figs. 1 & 5, b.br.s ) ; it is the ossification of intermuscular septa, and 
forms a sort of isthmus ; it does not answer to the “uro-hyal” of the Bird, which 
corresponds to the “ basibranchial” bar of the gill-bearing tribe, but should rank with 
the lateral rays and be called the “ basibranchiostegal.” Nearly twice as many rays 
proceed from the first branchial arch (Plate VI. fig. 3) ; these are pointed, flattened, 
arcuate ossicles, attached to the fore edge of the branchial bar; their direction is 
forwards and a little inwards. There is a single row of them on the first and fourth 
arches, but there are two rows on the second and third arches (fig. 4) ; these are so 
arranged as to form a colander through which the water is strained, as it is incessantly 
sent through the branchial clefts. In the Tadpole, as I have recently shown, similar 
structures commence on the branchial arches, which become conical elevations of cellular 
tissue, densely covered with tufted branchiae, and they serve both for straining and for 
respiration. In the typical Teleostei they only partially grow into bony rays even on the 
first arch, but form little mounds covered with bristling teeth. The merely fibrous bones 
are thus easily classified by considering their relation to the parts beneath ; they belong 
to the skin and its multifarious ingrowths ; the osseous matter has ceased to be formed 
from their outer surface, leaving the skin thoroughly differentiated from the skeleton 
within. These parts have attained a higher morphological condition than in the “ Ga- 
noids;” but they cease at this, their culminating point, and do not reappear, many of 
them, at least, even in the metamorphic “ Ichthyopsida.” Those bones, however, which 
do reappear in the higher classes become very constant, and are never lost sight of again 
even in our upward march to Man. 
The bony plates now to be considered (the deep laminae) possess a peculiar meta- 
morphic potency; for wherever they fasten themselves upon a cartilaginous rod or 
