DEVELOPMENT OE THE SKULL IN STURGEONS. 
161 
A. Ectocranium of the young Sturgeon (Acipenser sturio). 
In this stage there are scarcely any proper “parostoses” and very few “ endostoses,” 
but the head is well covered with “ dermostoses ” that are simply the ordinary Ganoid 
scutes (Plate 15, figs. 10-12), brought more or less into relation, both in form and in 
number , with the underlying endo-skeletal structures. 
Of course, the homology of these scarcely altered superficial scutes with the special 
deep laminae of bone that are so completely dominated by the endocranium in higher 
types, is imperfect and partial. These scutes present us with too much or too little 
when we are looking for the normal, highly specialised “ investing bones ” of the 
higher types; their inner layer, only, can correspond with those bones, and that is but 
imperfectly related to the parts within. Moreover, it is only in certain regions that any 
strict comparison can be made : this is in those cases where some unusually large scute 
has starved out its neighbours and has become the roof or wall of some particular part 
of the skull or face. Nevertheless, for the sake of uniformity of language, I shall call 
that scute which more perfectly than any other covers the nasal capsule, the nasal (n f .) ; 
the large plates over the hemispheres, the frontals (/ v .) ; those over the mid-brain, the 
parietals (p'.) ; and those over the auditory ledge, the squamosals ( sq 1 
In the face it will not be difficult to seize upon the true meaning of certain ichthyic 
bones, namely, the “operculars ;” and the “splints” (parostoses) that are applied to the 
highly specialised pterygo-quadrate and mandibular apparatus will also, by comparison 
with like parts in other Ganoids, and in the Teleostei , be interpretable. 
The round swollen head of the larva (Plate 12) is now changed into a long wedge- 
shaped recurved rostral structure (Plate 15, figs. 10-12),and this structure is invested now 
by solid ganoid plates in great number; but only certain of these can be pitched upon 
as deserving a special name.* The number of bones covering the snout is very great; 
none of these can be called “ premaxillary,” only one can even by courtesy be called 
“nasal” (figs. 10, 11, nf whilst below (fig. 12) several bones contend for the name of 
“ vomer.” The eye is protected by supra-, post-, and sub-orbitals (s.ob., pt.ob ., su.ob .); 
the post-orbitals run back and become temporal scutes, the chief of which is called 
the squamosal ( sq'. ). A very fine scute lies over the opercular region and is the true 
opercular (op.) ; under it there are two rugged, squarish plates—these are the sub- 
opercular and inter-opercular (s.op., i.op.) ; the pre-opercular (Plate 16, fig. 1, p.opf as 
in Lepidosteus, is a very small “ parostosis,” applied to the side of the quadrate region 
of the suspensorium. 
The orbital rim and eye (fig. 10) are small; the nasal pouch has a small upper, 
and a large lower opening; these openings are obliquely placed, so that the upper is also 
the foremost space. These capsules keep close to the antorbital region; in Lepidosteus 
they are carried to the end of the long beak. Behind the main post-orbital, and at 
* I must refer my readers to the views of those excellent “ experts ” whose works are referred to in 
the Bibliographical List, especially to those of Professors Huxley, Traquair, and Bridge. 
MDCCCLXXXII. Y 
