786 
ME. W. K. PARKER ON THE STRUCTURE AND 
(the pterotic, pt.o.) ; this latter bone can now be seen in the “ lateral cerebellar fossa,” 
on its front face *. 
A deep fossa partly subdivides the periotic mass from the occipital ring on the inside 
(fig. 3, op., e.o.) ; but the osseous centres are here not quite accurate landmarks of mor- 
phological regions, as the four periotic bones fail to wall-in the upper, outer , and lower 
portions of the auditory labyrinth : this is a reptilian state of things. 
The prootic (pro.) is the earliest and, as a correlate of this, by far the most potent 
osseous growth of the four. The crested fore edge of the periotic mass helps, in front 
of and above the labyrinth, to enclose the cerebral mass ; but the lateness of the small 
pterotic allows this cranial wing, both in the Bird and in the Mammal (see Huxley, 
op. cit. p. 245, fig. 97, Fr.O., which shows this part in the skull of a Beaver), to 
be ossified by the early prootic ; whereas in the Osseous Fishes the massive cranial wall 
is very greatly indebted to the pterotic, which is in them the largest of the periotic bones. 
The prootic of the Fowl (Plate LXXXIV. fig. 7, pro.), besides the scooped anterior crest, 
the front of which is notched for the trigeminal nerve (5), forms a large, backwardly 
turned, oblong plate, pierced along its middle for the compound seventh nerve. It is 
bifurcate above, where it creeps over the inside of the ampulla of the anterior canal, 
and also passes, in its posterior fork, to the other end of the arch ; here it encloses the 
ovoidal pterotic plate (; p.t.o .). The opisthotic (fig. 7, op.) is now a semielliptical wedge 
of bone, with its straight margin separated by a very narrow line of cartilage from the 
prootic, and its crescentic margin lying as a bank in front of the occipito-otic fossa ; the 
line of separation between the opisthotic and the exoccipital (e.o.) is a very narrow band 
of cartilage. 
The complex basisphenoidal region has now only one osseous centre, which, mace- 
rated away from the rest of the cranium, forms a most perplexing bone to one who has 
not followed its earlier history. Seen from below (Plate LXXXIV. fig. 6), we have 
its rostrum (r.b.s.), its great posterior wings ( pr.p .), and its basitemporal floor (b.t.). 
Towards the end of the rostrum the low anterior pterygoid processes (a.p.) look down- 
wards and forwards, and are covered with a meniscoid plate of cartilage. Behind them, 
at the mid line, the eustachian tubes open into one common vestibule, floored by the tips 
of the coalesced basi temporals. A notch, filled with tense membrane, is seen between 
the great upper wings and the basitemporals — these parts, in their meeting, not forming 
* This fourth, periotic bone (the so-called “ mastoid,” in the Eish, of Cuvier and Owen) was first determined 
by me, in a note to Professor Huxley, who spoke very cautiously about it at first (see his £ Elem. Comp. 
Anat.’ p. 188). In my former paper I described this hone (which is very common in the Bird-class) as the 
epiotic (see Plates xi.-xm., ep.) ; hut it is too far forward for that hone ; the true “ epiotic ” (see Eirst Paper, 
Plate vm. figs. 4 & 8, op.) is formed over the crown of the posterior semicircular canal. My error of taking 
the little pterotic for the epiotic misled me in naming the latter, which I supposed to he an outer ectosteal plate 
to the opisthotic element; and in the Eirst Paper it will be seen that the letters op. indicate both a postero- 
internal (true opisthotic) and a supero-external (true mastoid or epiotic) patch of bone. I am indebted to Pro- 
fessor Huxeey for leading me to see which was the true epiotic ; then the pterotic unconsciously assumed its 
true name. 
