DEVELOPMENT OF THE SKULL IN TIIE SALMON. 
103 
the skull, its bony matter ending abruptly in front. So much cartilage as is here 
ossified belongs, indeed, to the occipital ring; but whilst ossifying the cartilage had 
grown forwards to join a retral growth of a similar character which had crept along the 
cranial ridge all the way from the ethmoid (Plate VII. figs. 3, 4, 9, 10, 11, and Plate 
VIII. figs. 1, 4, 5, 6). This solid, subcarinate roof to the “great fontanelle” makes 
the endoskeletal skull of the adult Salmon very different to that of the adult Frog 
(“ Frog’s Skull,” Plate IX. fig. 6), which is barge-sliaped, and has a very imperfect 
“ deck.” These two types of skull do, however, conform to each other more than would 
seem at a hasty glance ; even in the Frog the annular ethmoid roofs in the great opening 
to some extent, and the superoccipital cartilage has grown to the anterior sphenoidal 
region. We have, moreover, in the Salmon the lateral fontanelles {pfo.), as in the 
Frog (Plate VIII. figs. 1 & 4). The rest of the square hinder part of the cranial box 
is almost entirely due to the impaction into the sides of the primordial cranium of a 
pair of very large ear-sacs, which coalesce very early with the investing mass, and send 
forwards from their anterior margin a lamina of cartilage which becomes the ali- 
sphenoid, and which is separately ossified. Anticipating the account of the earlier 
stages, I may say that the auditory sac is enshielded by cartilage from the outside, and 
is never totally encased as in the Frog. Also the huge size, in the young, of the 
semicircular canals causes upgrowths and swellings, according to their form, in the 
cartilaginous shield ; and more than this, for the skull of the Fish, especially behind, 
is related to muscular masses, hence apophyses have to grow out for their attachment 
and leverage. 
The eye reads all this in merely looking at the end view of the skull (Plate VIII. 
fig. 8), which shows a curious piece of architecture, the keystone of which and the 
lesser and greater wings thereof are produced and snagged. Five bony buds were 
during the first season grafted upon each swelling ear-sac, and they have transformed 
those simple encasements into the angular, ridgy, and winged mass which I have por- 
trayed in figs. 1, 2, & 8 in Plate VIII. All these, save one, can be seen from the 
upper surface (fig. 1); they are the “ sphenotic” ( sp.o .), the “ pterotic” {pt.o.), the 
“epiotic” {eft.), and the “opisthotic” {op.): the “prootic” {pro.) can be seen from 
beneath (fig. 2), partly sliced away from the outside (fig. 3), and in transverse section 
(figs. 4 & 5). The inner view (Plate VII. fig. 4) and the outer (Plate VII. fig. 3, pro.) 
are most instructive as to the most constant of the periotic centres. All the figures show 
how massive the periotic cartilage and bone becomes, and yet the labyrinth is only very 
partially imbedded in the mass. I borrow from the study of a prior stage the fact that 
the prootic commences in the thin anterior edge of the shield behind the exit of the first 
division of the fifth nerve, but enclosing the second ; also that the “sphenotic” begins 
over the ampulla of the anterior canal, the “ pterotic” over the ampulla and arch of 
the horizontal canal, the “epiotic” over the arch of the posterior canal, and the 
“opisthotic” over its ampulla. 
The prootics nowhere display such curious and unlooked-for characters as in the 
mdccclxxiii. p 
