DEVELOPMENT OF THE SKULL IN THE SALMON. 
129 
I have termed “ culmen cranii ;” then it stretches backwards as far as to the occipital cinc- 
ture ; now it covers only the first and part of the second cerebral division (Plate IV. figs. 
1 & 4). The “ great fontanelle,” which in the adult is covered in and reduced to two 
small lateral rudiments over the postsphenoidal region, is now a most elegant heart- 
shaped and large space (Plate IV. fig. 2, fo.), through which, in the dissected skull, 
half the basal region can be seen. Even in the adult Frog (“ Frog’s Skull,” Plate ix. 
fig. G, fo.) there is as much open space, for it reaches further forwards, although more 
covered in behind. If it be well considered, this expanded fontanelle of the young Sal- 
mon is essentially like that of the adult Polypterus (Traquaik, op. cit. plate G. fig. 2), 
which is bounded by a limiting cartilage that runs behind into the postfrontal part of the 
ear-sac ; this cartilage is like a wall-plate which has had part of the roof removed from 
it above, and part of the wall taken from beneath it. The creeping backwards of the 
ethmoidal “ tentorium ” at the mid line causes the emargination in front of the fonta- 
nelle ; and each “ funis tentorii ” is continuous behind with the sharp cartilaginous crest 
that arises as an upgrowth from the auditory sac, subparallel with the elegant curve of the 
“ anterior semicircular canal.” Reference to the profile view of the skull (Plate IV. fig. 1) 
will satisfy us that we have in this peculiar band the cartilaginous pith of that remarkable 
“supraorbital bar,” which I'have described in the first stage (Plate I. figs. 1, 2, 3, 4, 6, 7, 
s.ob.), which stretches from the nasal to the auditory sac. On account of their origin in 
the early embryo they may be called the “supraorbital bands” (, s.ob .) ; they have no distinct 
counterpart in the Frog or Fowl, but the retral part of the roof has a very evident 
counterpart (see “ Fowl’s Skull,” Plate lxxxiii. figs. 2, 4, 5, growing backwards from eth.). 
But the roof-cartilage is well developed in Birds generally; in the “ Struthiomdce” it is 
ossified separately from the “ pars perpendicularis” (“ Ostrich Skull,” Plate viii. figs. 
3 & 10, etli. p.e.), whilst in the Cassowary (ibid. Plate xiv.) it is enormously developed, 
and becomes the well-known “helmet.” I need scarcely mention the free roof-growth 
in Sharks, both ordinary and “ Chimseroid,” and also in the Sturgeon. 
Returning to the “ trabecula;, ” we find that they gradually narrow backwards towards 
the pointed apex of the cordiform pituitary space ; they thicken as they become narrow, 
and elbow out strongly before they apply themselves to the upper surface of the now 
rounded apices of the investing mass, from which they were very distinct in the speci- 
men figured. 
The pituitary body is let down through the cranial floor (Plate IV. figs. 2-4, py.) on to 
the subcutaneous parasphenoid ; but it occupies very little of the open space, which is 
further enlarged by free communication with the “posterior basicranial fontanelle” 
(Rathke), the gap in which the apex of the notochord lies. To this latter space the 
orbital muscles (figs. 2 & 3, o.m.) converge and lie (see Plate V. fig. 1, o.m.) upon the 
parasphenoid; at present there is no “prootic bridge” covering them above; the apex 
of the notochord lies upon their “ raphe.” The “ investing mass” ( i.v .) is now largely 
confluent with the auditory masses ; in front each moiety is separated from the prootic 
region by a rounded notch, and behind each side is developed into two lips ; those below 
