DEVELOPMENT OF THE SKULL IN STURGEONS. 
175 
angular, especially where the ectosteal tract exists. Only the first three have hypo- 
branchials (Plate 18, figs. 5 and 10, h. hr 1-3 .) ; these begin much larger than the hypo- 
hyal and then lessen backwards ; they are flat, finger-shaped, and not ossified. 
Another addition of cartilaginous segments has taken place below; in the young 
Sturgeons (Plate 14, fig. 5, and Plate 16, fig. 5) there was only one basi-branchial 
carrying the three first arches. Now, three new smaller segments have appeared 
(6.5r 4-6 .), the last of these is wedge-shaped, and belongs to a suppressed arch, for it passes 
behind the pedate end of the arrested fifth arch; the foremost piece belongs to three 
arches. 
These piers are compressed, and the first, which belongs to three arches (b.br 1 ^ 3 .), is 
nodose. 
Sixth Stage.—Skull of an unusually large* Sturgeon (Acipenser sturio) (in the 
Hunterian Museum). 
This very valuable specimen of a much older individual, shows some very important 
modifications, which have, as it were, an upward look towards the Holostean Ganoids 
and the Teleostei. 
In the last stage the pterygo-quadrate cartilages were even more perfect than in 
the young of 7 or 8 inches in length. In this specimen, however, the “ parostoses” 
have caused a considerable amount of absorption of the hyaline cartilage, just as the 
dentary, normally, causes Meckel’s cartilage to shrink and even become absorbed; 
but I cannot find any evidence of a direct ectosteal relation of the bone to the 
cartilage. The pterygoid bone (Plate 17, figs. 12, 13, jig.) gets over to the outer side 
over the convex inner margin, and causes the absorption of the cartilage in that 
region. Moreover, the jagged bony tract which represents the mesopterygoid, and 
which in the last was at the antero-inferior edge of the cartilage, is now (Plate 17, 
figs. 12 and 13, ms.pg.) a sharp wedge of bone filling up the space between the forks 
of the pterygoid on the inner side (fig. 13), and is seen as an oval plate of bone nearer 
the top than the bottom, on the outer side (fig. 12, ms.pg.). These differences are not 
all due to age ; there is a considerable amount of variation in individuals of the same 
age in these types, where the sub-cutaneous and sub-mucous bony tracts are but 
deeper scutes, and where the chondro-skeletal regions are so generalised and, as it 
were, hypertrophied. 
With regard to the buried scutes, we see a right and left variation in this same 
specimen (Plate 18, figs. 1 and 2, the right and left orbital region in this large skull). 
Here there are no proper ectosteal prefrontals (ecto-ethmoicls), orbito-sphenoids, alb 
sphenoids, or prootics ; but, since the last stage, parosteal tracts have appeared in 
all those places; they are deep “ scutes,” not shalloiv endo-skeletal elements, and are 
the prophecy, so to speak, of the special “ ectostoses” seen in the Holostean Ganoids 
and the Teleostei, and in all the types above them. Bound the thick antorbital 
* Even this specimen was not half the size this species sometimes attains to. 
