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MR. W. K. PARKER OK THE STRUCTURE AND 
The arch of the anterior and horizontal canals, and the vestibule (a.s.c, h.s.c ., vb.) are 
laid open, and below, in the great open “ meatus interims/’ the ganglion of the 7th and 
8th nerves is still seen. 
The hyomandibular (Jim.) is here articulated by its convex head with the concave 
surface of the auditory capsule, and looks like the proximal segment of a large limb 
joined to its own limb-girdle. This is correlated with the downgrowth of the mouth, 
whose own arch has been carried far away from its cranial attachment. Here we see 
the symplectic, inter-hyal, and proximal part of the cerato-hyal (sy., i.hy., c.hy .); and 
behind these, near the middle, the lower part of the first and second branchial arches 
(Ji.br 1 ., c.br . 3 , h.br 2 ., b.br.). 
Section 15. —In this section (Plate 15, fig. 7) the posterior canal (p.s.c.) is laid open 
throughout its whole extent, and here the capsule (cm.) is re-acquiring an inner wall. 
Below, it is joined to a very solid basal plate (iv.), right and left, the moieties of this 
plate clip the large notochord (nc.), which is only partly embraced by them. 
Parts of the first and second branchial arches (jp.br 1 ., e.br 1 ., c.br 1 ., c.br 2 .) and some 
gill papillae come into view here. 
Section 16.—In this view (Plate 15, fig. 8) the back wall of the auditory capsule (ait.) 
behind the posterior canal is shown; here the massive basal plates (iv.) are growing 
upwards to form the occipital arch (e.o.), and are separated from the auditory capsules 
by a large chink. Here the 9th and 10th nerves (IX., X.) are seen growing from 
the hind-brain (C 3 .), forming their ganglia and giving off their trunks. Part of the 
gill arches (hr.) and their gills are shown on each side of the pharynx. 
Section 17.—Here we see (Plate 15, fig. 9) that the occipital arch (e.o.) is imperfect 
above ; below, each mass of cartilage (iv.) cleaves closely to the huge notochord (nc.) 
the arch is produced into the angular processes on each side that project from the 
auditory capsules (see fig. 13). 
Fourth Stage .— Young Sturgeons (Acipenser sturio), to 8 inches long. 
In this stage the Fish is completely metamorphosed, and the only important change 
which takes place afterwards is immense increase in size, and the addition of certain 
bony centres, both parosteal and ectosteal. 
I have had no intermediate sizes between Sterlets 7 lines long and Sturgeons 
7 inches long, but in Lepidosteus , another Ganoid (the subject of my next paper), two 
instructive stages come in at this point, and make the interpretation of this lower 
type of skull easy. 
In larval Lepidostei the size of my largest larval Sterlets, namely, about 15 millims., 
the azygous intertrabecula has already filled in the space between the trabeculae, in 
front; and in specimens already like the adult, and 1 inch long, the trabeculae have 
developed their cornua, and the intertrabecula has shot forwards as a long pre-cornual 
rostrum. At that stage the endocranium of Lepidosteus is extremely Acipenserine, and 
explains, and is explained by, the skull of this stage in the young Sturgeon. 
