102 
MR. W. K. PARKER ON THE STRUCTURE AND 
plate they, as it were, devour it, and convert it into their own substance ; they have to 
be considered in connexion with the tracts of hyaline cartilage, which they are ever 
tending to obliterate. 
The skull and face proper, deprived of the investing bones, is a very complex struc- 
ture, a box above and a crate below ; the two divisions will be best understood by con- 
sidering them apart. 
And first the box itself (see Plate VIII. figs. 1, 2, & 8), which is a compound structure, 
formed behind of axial parts, and before of facial. But, besides the axial and facial 
elements, there is to be considered how much is due to what the great ear-sacs super- 
add, and also what is superadded by the facial elements to the nose-sacs, which help to 
build their crypts ; the eye-sacs are free, but are attached by a short cartilaginous pedicle 
(Plate VII. fig 3, o.pd.). There is one part of the skeleton which is truly azygous ; and 
this is a primary, fundamental part, a part to which all the axial structures apply them- 
selves; this is the “notochord.” In the adult skull there is only one bone formed 
upon this fundamental part, the basioccipital ; but the bony sheath of this axis acts upon 
symmetrical cartilages that appear very early, one on each side of the notochord ; these 
are called, together, the “investing mass.” This “investing mass” is the direct con- 
tinuation of that part of the embryo which is so early segmented into the vertebral 
rudiments, yet itself, still closely embracing the azygous axis, is under the controlling 
influence of some force which prevents further segmentation. A quasi-vertebral ring 
or arch is, however, formed, the “ occipital arch” (Plate VIII. figs. 1, 2, 7, 8) ; and this 
ring, seen from behind (fig. 8), has all the appearance and many of the characters of a 
vertebra. Only four of the bones seen from behind belong to this segment, the basioc- 
cipital ( b.o .), the “ exoccipitals” ( e.o .), and the “ superoccipital” (s.o) : the three outer- 
most pairs belong to the auditory capsule; they are “ otic elements.” But the “ basioc- 
cipital” bone does not utilize all the “ investing mass ;” the notochord retires during 
growth, and the rest of the investing mass is ossified (not thoroughly) by the foremost 
of the otic bones, the “ prootics.” One remarkable change in the investing mass, as 
a whole, is the growth downwards of a lamella on each side, thus forming a covered 
archway ; for in front of the retiring notochord the moieties of cartilage meet, and this 
viaduct is floored by the submucous bone which has been removed (Plate VIII. fig. 2), 
the “ parasphenoid.” All the true axial parts of the skull cease at the fore edge of the 
investing mass behind the pituitary space (jpy .) ; all the rest has a facial foundation, is 
built on the “ trabeculae,” or has a secondary character as a development in the cranial 
wall. The compound eighth nerve (Plate VII. figs. 2, 3, 4, s, and Plate VIII. figs. 
2 & 3, s) passes out of the skull between the postero-internal face of the ear-sac and 
the investing mass ; it pierces the exoccipital in the adult. These latter bones present 
flat zygapophyses for the “ atlas,” which is also joined to the basioccipital, a notochordal 
‘buffer” remaining between the two. The “superoccipital” (Plates VII. & VIII., s.o.) 
is a massive bone ; it does not, as in the “ Sauropsida,” receive any of the “ anterior audi- 
tory canal ;” and it extends halfway along the roof of the square postpituitary part of 
