140 
ME. W. Iv. PAEKEE ON THE STEITCTUEE AND 
anterior sphenoid occupied by the “ culmen cranii ” is membranous, and the membranous 
orbito-sphenoids of the Salmon are represented by cartilage in the Mammal. Again, 
the alisphenoids of the young Salmon are scarcely more free from the ear-cartilage than 
the laminar growth of the prootic region in an average Mammal (see in the Beaver, 
Huxley’s ‘Elem.’ p. 245, fig. 97, as., pro.)-, but where the alisphenoid of the Mammal 
thins out above, there in the Fish it has a thick selvedge, connecting it both with the 
ethmoidal roof and the periotic capsule. The basisphenoidal region is also in a most 
rudimentary and, as it were, fragmentary condition ; for there is a feeble prepituitary 
portion, an open sellar space, and the cartilage ( basilar ) which should form the posterior 
clinoid wall, the postpituitary region, and the spheno-occipital synchondrosis, — all this is 
trespassed upon by the “ prootic bones,” which borrow it to form their curious basi- 
cranial bridge. 
Relatively to the rest of the skull the auditory capsules are very large ; their centres 
of ossification are curiously placed — one on the anterior margin behind and above the 
main part of the fifth nerve, and perforated by the posterior branch, and the other four 
forming a rfi/A^-angular series along the most projecting part, covering the galleries of 
the labyrinth (Plate V. fig. 7). The first is the “prootic” (pro.)-, the second, which is 
over the ampulla of the anterior canal, is the “sphenotic” (sp.o.) ; the third, over the 
ampulla and arch of the horizontal canal, is the “ pterotic ” ( pt.o .) ; the fourth, over 
the arch of the posterior canal, is the “ epiotic ” (ep.) ; and the fifth, which is over its 
ampulla, is the “ opisthotic ” (op.). Of these Professor Huxley is responsible, as to 
nomenclature, for three, the “ tria ossicula” of Kerkrixgius, who described them in the 
human skull (see ITuxley’s ‘Elem.’ p. 153); the other two which I contend for are 
the “ sphenotic ” and “ pterotic.” At present these bones are in their infancy, the car- 
tilage is whole beneath them ; it still retains the impressure and form of the elegant and 
large elements of the labyrinth. The most exquisite part of this cartilaginous capsule 
is seen beneath the “ tegmen tympani” and facet for the “ hyo-mandibular this is the 
ovoidal pouch for the “ sacculus,” which lies between the foramen for the “portio dura” 
(7") and that for the compound eighth nerve (8). The antero-superior part of this 
“ saccular recess ” is occupied by an oval fenestra, the primordial deficiency already 
described (see Plate II. fig. 4) ; it does not, however, persist, but closes as in the third 
stage of the Frog, and does not reopen as in that air-breathing type (see “ Frog’s Skull,” 
Plate iv. fig. 7, and Plate v. figs. 1 & 4, au., st.). That out-bent part of the trabeculae, 
the apical part which joins the “ investing mass,” is still present, but is a temporary 
structure ; thus the three anterior facial arches all escape away from their primary rela- 
tions to the base and sides of the cranium. The lowered position of the pterygo-palatine 
and mandibular arches is shown in the next figure (fig. 8; the rest of the face detached 
from the skull, fig. 7). 
The “ superoccipital ” region is now invested by an ectosteal lamina, and the “ foramen 
magnum” is bounded laterally by a pair of crescentic ossicles, the exoccipitals (Plate 
Y. figs. 6 & 7, so., eo.). 
