104 
ME. W. Iv. PAEKEE ON THE STEHCTUEE AND 
Osseous Fish ; they beguiled both Cuvier and Owen into the supposition that they were 
the great wings of the sphenoid. They surround part of the fifth nerve, send their 
osseous matter trespassing across the “investing-mass bridge,” and also down into its 
deep descending keels — those most ichthyic protectors of the orbital muscles (Plate VII. 
figs. 3 & 4, and Plate VIII. figs. 2, 4, & 5, pro.). A broad synchondrosis exists between 
the prootics and the upper bones ; and below this, on the inner side (Plate VII. fig. 4), 
the prootic is trilobate as it embraces the hinder division of the fifth nerve (5 4 ) and the 
“ portio dura” (7"). When the outer face of the skull has been sawn away (Plate VIII. 
fig. 3, ct.s.c., pro.), it can be seen that the ampulla of the anterior canal is partly imbedded 
in cartilage, and that the tubular communication of the ampulla with the rest of the 
labyrinth really burrows its substance and reenters the skull by the recess which opens 
outwards for the second division of the fifth nerve (5 6 ). 
The bony bridge made by the prootics from the fore part of the investing mass arti- 
culates in front with the “ basisphenoid ” (Plate VII. fig. 4, pro., b.s.), and behind 
with the basioccipital (b,o.). The two supero-external pieces carry the long concave 
facet for the wide head of the hyomandibular (Plate VII. fig. 3, and Plate VIII. fig. 2, 
sp.o., pt.o.), and together form the “tegmen tympani,” or rather its ichthyic counter- 
part, for here is no tympanum. The “ sphenotic” is only half the size of the “ pterotic,” 
but it is a solid bone (see sections, Plate VII. fig. 11, and Plate VIII. figs. 4 & 5, 
sp.o.); it forms a strong forthstanding spur protecting the orbit, whilst the pterotic 
sends a similar spur backward for muscular attachment. Neither of these bones can 
be seen from the inner face of the skull (Plate VII. fig. 4) in a direct lateral view, nor 
can the “ epiotic” or “ opisthotic.” The Bird differs from the Osseous Fish, whilst 
agreeing much with it, for it has all the five periotic centres ; but the cartilage is not so 
thick as in the Fish, and it is entirely ossified; the “sphenotic” and pterotic are both 
small, but the former (“postfrontal”) has the same shape as in the Fish and the same 
relation to the “ alisphenoid ” (see “Fowl’s Skull,” Plate lxxxiv. figs. 6, 8, 13, 14, 
p.f., a.s. ; compare especially fig. 14 with the section, fig. 4, in Plate VIII. of this 
paper). 
In the Fish the huge sweep and vertical position of the semicircular canals makes the 
membranous labyrinth bear a much larger proportion to the cranial walls than in the 
Bird, in which the fore part of the periotic cartilage rapidly modifies itself in relation to 
the membranous cranium, and the anterior canal leans backwards ; thus the “sphenotic” 
region grows out free from the labyrinth, and its ossicle looks like a mere epiphysis on 
the “ alisphenoid.” In the Bird the cerebellum excavates the cranial wall by its pro- 
jecting lobe, and thus brings the little pterotic into view within ; whilst the opisthotic 
wedges itself in between the prootic and exoccipital, and is greatly modified to encircle 
the mouth (“ fenestra rotunda”) of the small cochlea (see “ Fowl’s Skull,” Plate lxxxv. 
fig. 3, op.,f.r.). The epiotic and opisthotic of the Salmon are both very backwardly 
placed in relation to the posterior canal (Plate VIII. figs. 1, 2, & 8, ep., op.); they 
correspond in the adult Fish with their condition in the newly hatched Fowl or Chelo- 
