408 
MR. W. K. PARKER ON THE STRUCTURE AND DEVELOPMENT 
The basioccipital (ho.) is an irregular lozenge, transversely placed; it is slightly 
emarginate in front, and mammillate for muscular attachment. 
It articulates with the basisphenoid in front (Plate 32, fig. 2, ho., hs.), with the ex- 
occipitals postero-laterally ; in front of these (Plate 33, figs. 1, 2, pro., op.) it receives 
the prootic and opisthotic. 
With the hind part of the basisphenoid (Plate 33, fig. 1), it forms a very consider- 
able hollow for the medulla oblongata. 
The strong sides of the arch, the exoccipitals (eo.), run their lower angle into the 
condyle ( oc.c .), whilst above the foramen magnum they meet and push out the super- 
occipital wedge (so.); they have coalesced with the opistliotics ; the 9th, 10th, and 
12th nerves pass out (Plate 33, fig. 2) close to the lower line of their ankylosis. 
The superoccipital also seems to be much larger than in the young ; this arises 
from its coalescence with the right and left epiotics (ep.). 
It is now a five-sided, transverse roof-bone, its outer or epiotic angles being some- 
what convex (Plate 32, fig. 1 ; Plate 33, fig. 2). 
The parietals (Plate 33, fig. 1 , p.) scarcely overlap the superoccipital ; the junction 
is by a harmony-suture, exactly where the prootic (pro.) begins. 
This main auditory bone has also coalesced with a true cranial element — the ali- 
sphenoid (cd.s.) ; yet the tri-racliate suture between the three periotics is persistent ; 
each element of the capsule is distinct from the other two, and yet each is fused to a 
neighbour-bone of the skull wall. 
Here a distinct cartilaginous sense-capsule is ossified by three centres, which dis- 
claim, as it were, their true affinity to each other, and lose themselves in the cranial wall. 
In the Mammals, in sharp contrast with these transformations, the capsule chondri- 
fies with but slight separateness from the cranial wall ; and then these three centres 
ossify the capsule, and completely enucleate it from the skull proper. This is best 
seen in Shrews, the lesser Bats, and the Whale -tribe. 
In the Bird these parts behave as in the Reptiles, but with this difference — namely, 
that the opisthotic and epiotic are very small, so that the labyrinth of the ear grows 
into the occipital arch. 
The otic elements first unite with their cranial neighbours, and then everything is 
ankylosed ; all the morphological writing is blotted out in that Class. 
These lateral and upper cranio-auditory plates are marked with the form of the 
enclosed labyrinth, as may be seen by comparing their wavy surface with the form of 
the earlier cartilaginous capsule. 
Infero -laterally, these plates are riddled with holes, mostly for the passage outwards 
of the cranial nerves. 
In the crescentic chink below the opisthotic region (op.) the exoccipital is perforated 
for the 12th, 10th, and 9th nerves : these can be seen on them emerging below, 
but better in the inside ; for externally they are partly hidden by that angle of the 
opisthotic which contains the hinder ampulla (p.sc.). 
