146 MR. W. K. PARKER OK THE STRUCTURE AND 
There are, now, seven cartilaginous arches on each side, the two first are large, the 
rest smaller; they are all out of sight, more or less, the operculum hiding the two first, 
and the gills those behind ; the clefts also are largely hidden from view by these 
structures. 
In the smaller specimens at this stage (fig. 4) the form of the two first arches can 
be seen through the skin, the pterygo-quadrate bar running within the upper lip, 
Meckel’s cartilage within the lower jaw, and the hyoid arch (hy.) inside the fore part of 
the opercular fold (op.). The opercular fold of the mandible is very limited on account 
of the closing in of the ventral part of the first cleft (cl 1 .), but its serial homology with 
the large next fold (op.) is manifest. Inside it is thickened by a mass of cells from 
the first liypoblastic pouch, and this mass, somewhat obscurely, shows on its surface 
two rows of short mammillate projections—arrested gill papillae. 
But the next fold, with its lining, is greatly developed (Plate 12, figs. 4-7, op.), and 
its inner face is covered with rather long papillae, like those which grow freely from 
the proper branchial arches (br.). Membranous, at present, this free fold of the hyoid 
arch is destined to contain three ganoid scutes, the upper of which (the opercular) 
will be one of the largest of the huge plates developed in this fish. 
The rich growth of simple longish papillae on the other arches hides them from 
view; these gills, however, are much shorter than those seen in the large embryos of 
Selachians. 
When the skin and the gills are removed from the side of the head in one of the 
larger larvae at this stage—9| millims. in length—then the form of the newly chon- 
clrified bars is seen (Plate 13, fig. 6). 
I have already remarked upon the Selachian appearance of the larvae at this stage 
(Plate 12, fig. 7); a dissection, like the one under notice, displays much more than 
superficial Selachian characters. Even in the adult the Sturgeon has not made much 
advance towards the Teleostean culmination of the fish-type, but the dermal scutes 
have been largely dominated, in the head, by the cartilaginous endo-skeleton; and in 
the face, behind the mandible, we find many of the bars ensheathed in their own 
“ ectostoses.” 
The hyostvlic type of skull, which is not seen in the generalised forms, such as 
Notidanus and Cestracion, but is common to Skates and ordinary Sharks, is in the 
“ Acipenseridse ” carried to its uttermost pitch of perfection, and here, in this minute 
larva, recently hatched, and scarcely chondrified, the Selachian type of skull is over¬ 
passed, and at least two additional segments are seen on each side. These additional 
joints in the highly subdivided and modified primary hyoid bar are expressly for the 
purpose of throwing the mouth out far away from the skull; the jaws are suspended 
on an extremely hypertrophied upper hyoid segment. 
This hypertrophy of the “ epi-byal ” segment is correlated with the atrophy of the 
uppermost, or suspensorial part of the pier of the mandible, which pier, even in this 
early larva, is below the middle of the hyoid arch Plate (13, fig. C>, pg.<p, mnhm ., c Jiy). 
