OF THE SKULL IN THE UKODELOUS AMPHIBIA. 
557 
a Selachian and the osteocranium of a Reptile. Several pairs of bony centres now exist 
that are intimately blended with, grafted upon, and by ingrowth destructive of, large 
tracts of cartilage. 
But still larger tracts are soft, and only one pair of bones occurs on the inferior arches 
that have any right to be called endoskeletal, namely the “ articular ” pieces (Plate 25. 
fig. 5, and Plate 26. fig. 5, ar.). 
On the whole this skull agrees well with that of any rather large larva of a Caduci- 
branchiate Urodele. 
True to the Amphibian type, there is neither a floor-bone nor a keystone to the 
occipital arch ; the tendency once shown to vertebral segmentation has now vanished, 
and the notochord, deprived of its bony sheath, has now become a mere thread in the 
basal cartilage. 
On each side of the fontanelle, in front, there is a tract of bony masonry (Plate 26. 
figs. 3, 4) ; but for the most part the parachordal and trabecular regions run to the 
frontal wall as one solid mass of cartilage. 
As compared with former stages, the fontanelle is less ; but that is due to the huge 
plate and “horns” in front, for the basal plate ( b.o .) runs no further forward than the 
occipital roof ; these two territories were extremely unequal (see last stage) (Plate 25. 
figs. 1 & 2). The upper fontanelle is much larger than the lower, the sides of the latter 
having grown into the floor considerably ; especially in front is the floor cartilaginous, 
and the two representatives of the many olfactory foramina of a Mammal are conspi- 
cuous with their nerve (1) from above. 
The exoccipital bones ( e.o .) are wider apart below than above ; but in both cases there 
is a clear tract of cartilage between them; the cartilage of the condyles ( oc.c .) looks 
downwards. Above, these bones run but little in front of a transverse line that might 
be drawn across the top of the foramen magnum ( f.m . ) ; but on each side the auditory 
capsules are largely hardened by them. Also below, shunning the median basal plate, 
the exoccipitals grow out into a large “ opisthotic ” shell, which floors most of the 
vestibular bulb (fig. 4). A little more specialization, and this plate would have become 
an independent opisthotic centre. 
Nearly all the posterior canal and its ampulla is invested by the upper part of the 
exoccipital bone ; therefore that part is an epiotic region of the bone. 
But the horizontal canal, which burrows the outer edge of the capsule, has its own, 
evidently independent, periotic centre ; this irregular shell of bone, which forms a rough 
tegmen tympani, is the “ pterotic.” 
The prootic is perhaps the most curious bone of all : it begins as a film on the fore 
face of the capsule, where the anterior canal and its ampulla is enclosed. During growth 
it finds its way down to the floor of the front part of the capsule between it and the 
plaster of newer cartilage which was derived from the basal plate, and which persists as 
a soft socket for the “pedicle” of the suspensorium ( pd .). But the prootic (which in 
the Batrachia often vicariously ossifies the alisphenoidal region) here forms a stony 
