636 
ME. W. K. PAEKER ON THE STRUCTURE AND 
chord ( nc .) is much reduced, but can be clearly seen on the upper surface of the skull 
floor, and through the cartilaginous basioccipital region below. Between the occipital 
condyles (oc.c.), which look backwards and a little inwards, there is a deeper crescentic 
margination below, and a shallower above. Three fourths of the parachordal region, 
behind, and almost all the trabecular in front, is underspliced by a huge dagger of bone, 
the parasphenoid (pa.s.), which is rapidly acquiring its adult characters. 
But as at first (Plate 57. fig. 2 ,pa.s.), the hinder margin is still concave. The “ guard ” 
is a large lozenge of bone, with the hinder angle produced considerably and the front 
part greatly ; this part, narrowing gently, runs up to the septum nasi, and there ends 
in a blunt point, which point is guarded by a transverse semicircular bone, the vomer (v). 
The exoccipital bone ( e.o .), which in the last had crept up to the parasphenoidal 
handle, has now crept over its edges, and has not only ossified the sides of the investing 
mass, keeping accurately to the boundaries of that part in front, but after hardening 
the submesial fossa it runs beneath the vestibule, so as to leave it only a selvage of car- 
tilage. Behind, the bone runs to the margin of the capsule, enclosing the 9th and 10th 
nerves (fig. 3. 9, 10) in a double foramen; in front it just touches the point where the 
apices of the trabecula and mandible coalesce (fig. 3, tr.,pd.). 
Above (fig. 2) the exoccipitals keep apart so as to leave a soft, square supraoccipital 
region (s.o.) ; behind it runs to the edge of the foramen magnum ( f.m .); externally it 
reaches to the posterior canal, and runs forward over the edge of that and the 
anterior canal, at once fringing the canals outside, and the “ fontanelle ” towards 
the middle. This upper part ends, curving outwards, in front of the ampulla of the 
anterior canal. All this bony matter began in the occipital arch behind, and has grown 
into the substance of much cartilage above and below (Plate 57. figs. 3, 4, and Plate 58. 
figs. 2, 3, e.o., pro.) ; it is a “ prootico-exoccipital ” — a double bony element. 
The breadth of the great trabecular floor is not yet lessened, and it runs full in width 
up to the fore margin of the ethmo-palatine bridge ( e.pci .), the front edge of which is 
semicircular, as the hind margin of the internal nostrils ( i.n .). The projection inside 
this notch shows the breadth of the trabecula ; there, however, it suddenly narrows in 
to form the subnasal laminae, the moderately alate base of the septum nasi (figs. 2 & 3, 
s.n.). The septum above (fig. 2) has no larger aliseptals than the Common Toad 
(Plate 54. fig. 3, al.s.), but the subnasal plates are very small as compared with the 
common kind (Plate 54. fig. 4, and Plate 58. fig. 3). This deficient nasal floor is a 
curious arrest of the cartilage, which is so profuse immediately behind this part. The 
hurried retreat of the quadrate condyle (q.) has curiously affected the nasal labyrinth; 
the large flappy cornua trabeculae (c.tr.) are now small crescentic wings, tilted in front of 
the nasal cavity. Where these are set on the septum (fig. 3, s.n.) is trilobate ; the median 
lobe is the arrested rudiment (pn.) of the long “ prenasal ” of the Elasmobranch fish 
and the bird, and the paired lobes ( al.n .) are the bud-like rudiments of the floor of 
the alee nasi of the higher Yertebrata. The cornua trabeculae {c.tr.) are curiously 
overlapped by the most highly developed form of nasal valve with which I am acquainted. 
