142 
MR. W. K. PARKER ON THE STRUCTURE AND 
chondrocranium is nearly as perfect as in a Skate. Yet every true Mammalian 
character is to be seen, well developed, and diagnostic. 
The cranio-facial axis has two bones in its hinder third, not much larger than 
the “synchondrosis” that separates them; these are the basioccipital and the basi- 
sphenoid (b.o., b.s.) ; all the rest forward is pro-chordal, and composes the presphenoidal, 
ethmoidal, and septal regions of the skull (p.s., p.e., s.n.). 
The septum runs forwards, between the folds of the alinasal cartilage ( al.n.) in 
front, a roundish part, perfect, or nearly so ; then a narrow isthmus unites this part 
with the proper septum nasi (s.n.), which passes, at present, without change into 
the perpendicular ethmoid (p.e.), and this into the presphenoid (p.s.). This large 
partition wall is a low triangle, the apex of which forms the rudiment of the crista 
galli (cr.g.). The fore part, above, is continuous with the nasal roof ( al.e., al.sp., al.n .); 
the hind part divides the two great olfactory fossae, with their hollow, cribriform 
floor (cr.p.). The basal part of this great wall is thick, this arises from the primary 
solidity of the intertrabecula. Where the a he nasi (al.n.) seem to end below, there 
they give off the recurrent cartilages (rc.c.), and these are strengthened by their 
special splints, the anterior paired vomers (fig. 8, v'.) ; behind these, the intertrabecula 
is supported by the large grooved vomer, proper (v.). From the crista galli (cr.g.) to 
the foramen magnum there is one continuous growth of solid cartilage, the fore part 
of which becomes the crest of the orbitosphenoid ( o.s .). Up to the sphenoidal fissure 
(V 1,3 .) the cartilage runs from the base to the top without any break save the 
foramen opticum (II.), which passage is now enclosed in the wedge-shaped orbito- 
sphenoidal bony centre (o.s.). This beginning of a large plate takes up all but the 
lowest part of the stem of the great orbital wing, but only reaches one-third of the 
way to the sinuous upper edge of the cartilage. Hence, a full fourth of the whole 
side wall has given way outwards, and the top of the cartilaginous wall reduced to one 
half its depth, forms an elegant archway over this breach ; it is a very perfect arch, 
but leans a little forwards. This doorway is only partly shut above by the ali- 
sphenoid (al.s.), the part which has been thrust out ; the lower half of this half- 
opened valve is ossified, and the lower edge of the bony part has a large notch in it, 
behind the middle ; this notch is finished by cartilage, and is the foramen ovale (V 3 .). 
The basisphenoid (b.s.) is growing into this tract of cartilage, and reaches further 
backwards than the alisphenoid. 
The cartilaginous side wall has then a second great archway larger than that caused 
by the out-thrust of the alisphenoid; here there is an actual suppression of the wall, 
but the space, which looks a little backwards, is filled in by the large ovoidal cartila¬ 
ginous auditory capsule. Over the capsule the cartilage is thinned out by pressure 
of the lateral sinus ( l.s .), which forms as perfect an arch as that over the alisphe¬ 
noid. Under the arched swelling caused by the anterior and posterior semicircular 
canals, which meet above, there is but a shallow concavity for the flocculus cerebelli. 
Behind this hollow, there is an unciform opening, with its convex margin behind ; this 
deficiency is caused by the “ recessus labyrinthi.” 
