480 
MR. W. K. PARKER OK THE DEVELOPMENT 
Binding on the fore edge of the two last, we see an angulated sub-crescentic bone, 
with its concave edge above, its angulated margin below, and its front point binding 
under the hind point of the preoperculare. This is the interoperculare (Plate 37, 
fig. 4, and Plate 38, fig. 5, i.op.). These three bones are the hyomandibular splints ; 
those now to be described belong to the cerato-hyal; these are the branchiostegals : 
they are beneath and within the great operculum (Plate 37, fig. 1, br.s.), and are 
attached in front to the cerato-hyal. Generally seven in number in the Teieostei, 
there are only three here, as in the Carp. These bones are narrow and falcate, with 
their concave margin above ; the uppermost is the largest; the lower is the least. 
The remainder of the bones which I have to treat of are intrinsic centres, or 
“ ectostosesthey will come under notice, now, in a description of the endocranium, 
most of which, however, is cartilaginous. 
The endocranium at this stage differs but little from that of the adult, in which, 
although the bony centres become dense and relatively larger, are yet not altered, 
either in their number or relations, to any appreciable degree. 
Jn this skull, the “prenasal rostrum/’ or intertrabecula, is as much developed as 
in the most specialised of the Selachians—namely, the “ Pristidae,” or Saw-fishes — 
much more than in the ordinary Skate (“ Boiidse ”). Here the length of this precranial 
region is, as compared with the cranial cavity, as 14 to 5, or nearly three times as 
long. In an old specimen the cranial cavity is only 2 inches long, and the whole skull 
12^ inches, or 2 to 10^; the brain has, relatively, retreated. Measured from the 
quadrate condyle, in this young stage, we get the same proportion as the measure¬ 
ment of the precranial to the cranial; in the old the prequadrate region is 9 inches 
long, and the post-quadrate 3-| inches. 
This remarkable pyriform, long-stalked skull owes its greatest expansion to the 
superaddition behind of the large ovoidal auditory capsules, and next to them to the 
greater size, in the young, of the mid brain; it soon narrows in over the small hemi¬ 
spheres. As in the last stage, the roof of the mid brain is membranous—this is the 
large circular fontanelle (Plate 38, fig. 1, fo.), the margins of which are very moderate 
bands of cartilage—the postorbital part of the superorbital bands (see Plate 30, fig. 8, 
so.b.). 
Below (Plate 38, fig. 3, py., pa.s.) there is a rather large and lanceolate pituitary 
fontanelle ; and inside the orbits (Plate 38, fig. 2, o.sfi.) there are the long “ orbito- 
sphenoidal fenestrse.” The upper fontanelle (fo.) takes up about a third of the 
roof, but it is not so long as either the fore or the hind part of the tegmen cranii 
(i t.cr .). Thus although this is rather a well-developed chondrocranium it has four large 
membranous deficiencies in it. 
The basioccipital (Plate 38, figs. 1-3, bo.) might be taken for the centrum of the first 
vertebra—it projects so far behind the exoccipitals (e.o.). The bony matter, which 
did occupy much of the sheath of the cranial notochord, is now mainly confined to its 
hind part, and forms a four-sided mass ; this mass does not run forwards into the rest 
of the notochord—at least on its upper surface (Plate 9, fig. 3, nc.). 
