162 
MR. W. K. PARKER OR THE STRUCTURE AND 
the antero-superior angle of the opercular, the small spiracle (cl 1 .) is seen ; inside the 
opercular the half-gill of the hyoid arch may be seen, transferred from its own arch, 
now required for suspensory purposes, and attached to the inner face of a specialised 
ganoid scute. The mouth and lips (fig. 12 , to ., u.l., l.l.) are now curiously modified, and 
this structure is extremely protusible; the opening is transverse, and crescentic when 
closed. 
The barbels (figs. 10, 12, hh.) are long and slender, now; they have lost their carti¬ 
laginous pith. 
I shall describe the oral parostoses with the cartilages to which they are attached. 
B. Endocranium of young Sturgeons f rom l\to 8 inches long. 
When the superficial bones have been removed we find an extremely solid chondro- 
cranium underneath (Plate 15, fig. 13, and Plate 16, figs. 1-4). In the last stage there 
was a continuous membranous fontanelle along the whole top of the head, the orbito- 
sphenoidal region was partly membranous, right and left, a small fontanelle existed 
under the pituitary body, and an open notch in front between the trabeculse. 
Now, the only membranous space is a small trilobate supraoccipital fontanelle 
(Plate 15, fig. 13, s.o.,fo.), not over the proper brain cavity but in an extension of the 
chondrocranium over the fore part of the spinal region. 
Everywhere there is the same intense hypertrophy of the hyaline cartilage, and in 
no part of the cranium, proper, nor in the auditory capsules, do true “ ectosteal ” 
plates graft themselves upon the cartilage—even in very old individuals; moreover 
there is no calcification of the surface-cartilage, such as is seen in the Selachians. 
Even now, in these young specimens, the actual size of the brain and brain-cavity is 
extremely small (Plate 16, fig. 2) in proportion to the size of the skull; which, 
measured to the end of the rostrum, is three times the length of the cranial cavity. 
Here we see the permanence of the early “ mesocephalic flexure; ” for besides the 
sudden loop formed by the mid-brain, represented now by the post-pituitary chink, 
which looks forwards, the solid ethmoidal region of the skull is bent gently, but steadily, 
downwards, before it rises to form the recurved rostrum. The orbits are very large, 
out of all proportion to the small, thick, cartilaginous sclerotics; the nasal capsules (ol.) 
are set in the sides of the hind part of the huge rostrum, the ethmo-palatine or ant- 
orbital wings of which are thick and twice swollen. The rostrum is composed of three 
tracts, answering to the three offshoots of cartilage that have grown so rapidly since 
the last stage from the end of the primary chondrocranial floor-bands—the short pro¬ 
chordal trabeculse. Seen from above (Plate 15, fig. 13) and in section (Plate 16, 
fig. 2, c.tr., i.tr.) the rostrum is convex at its edges, and gently concave in the 
middle; but below (Plate 16, figs. 3, 4) it is like a sagittate leaf—thick and succu¬ 
lent—with a very solid convex mid-rib and thickened margins; here the margins 
are the cornua trabeculse and the mid-rib the intertrabecula [c.tr., i.tr.). The deepest 
