146 

ON THE STRUCTURE OF THE EELS SKULL 
ES skull of the Eel is much less specialised than 
that of most other Osseous (Ze/eostean) fishes. 1 
was made aware of this many years since whilst pre- 
paring skeletons of the common kind (Azguzlla acuiéc- 
vostris), and of the conger (Murena conger). After- 
wards, when Prof. Huxley’s “ Croonian Lectures” (Proc. 
Roy. Soc.) came into my hands, the importance of the 
aberrant structures of this type of skull was shown to 
me ; and since that time I have been on the watch for 
further opportunities for dissecting and working out 
both this type, and also that of the Amphibia, which 
it serves to illustrate. In a few weeks’ time I shall 
be able to make myself understood with regard to those 
morphological changes which take place in the vertebrate 
skull as it passes from a low Ichthyic into the higher 
Amphibian type. This will be done by the illustration 
and description of the frog’s skull in the forthcoming 
part of the ‘ Philosophical Transactions,” an abstract 
of which paper has already appeared in these columns. At 
present the nomenclature of the parts of the cranium 
and face of the fish is in a state of painful confusion. I 
shall not, however, trouble the student with confusing 
references, but continue to use those terms which he will 
find in my other morphological papers. I may, however, 
remark that these differ in some instances from those 
used by Professor Huxley, for instance, his “ squamosal” 
is my “pterotic” (see “Elem. Comp. Anat.,” p. 188). 
This is a bone called “mastoid” by Cuvier, and this 
term was adopted by Prof. Owen. These anatomists came 
much nearer the truth than my friend ; but the bone only 
represents fav? of the human “mastoid ”—its antero- 
superior region. Again, the terms for the palato- 
pterygoid arcade are very confusing ; Cuvier’s “internal 
pterygoid,” also called ento-pterygoid by Owen and 
Huxley, does not correspond to the internal pterygoid plate 
, of man and the mammalia generally, but to a third piece, 
| which I call meso-pterygoid, and which occurs in a young 
pig’s and in a young fox’s skull in my collection ; I have 
also found it in the palate of all sorts of birds, except the 
fowls and Struthionida. The true representative of the 
human internal pterygoid is, in fishes, called “ transverse ” 
_ by Cuvier ; most correctly the “ pterygoid” by Owen; 
confusingly the “ ecto-pterygoid” by Huxley. I drop 
the frequently misapplied terms, “ecto-” and “ ento-ptery- 
goid,” altogether, and call the true “transverse bone” of 
the reptile—never seen in fishes—the “transpalatine.” 
Most of the other terms used by me agree with those used 
by Prof. Huxley in his ‘‘ Elements.” 
There is, however, in the hyoid arch one segment 
which requires its name,—that given to it by Prof. Owen— 
to be changed; I refer to that lump of cartilage which 
. becomes segmented off from the lower part of the hyoid 
, cornu by a joint cavity, and which has two ossifiic centres. 
| This has been called the “basi-hyal ;” but it is merely a 
distal and not a basal bone, the key-stone being the 
“slosso-hyal,” which passes into the basi-branchial bar. 
I would call it the ‘‘hypo-hyal,” as it is the manifest 
“serial homologue” of the “ hypo-branchials.” All these 
things I hope soon to make plain in a paper now in 
hand, on the “ Structure and Development of the Salmon’s 
Skull.” My materials at hand, from which I have studied 
the eel’s skull, are the adult conger’s skull, that of a 
small Afurena (? species), and the heads of large and 
small common eels. The smallest of these are the gift of 
Mr. F. Buckland ; they measure 2 inches 8 lines in length. 
The cranium of the eel is long, triangular, and depressed, 
the nasal region being very pinched and narrow, whilst 
the occipital is expanded, and sends out over-hanging 
outgrowths, — backwardly projecting crests, which are 
continuous in the conger, but distinct spurs in the eel. 
These crests in the eel are formed by the super-occipital 
at the mid-line; then a pair from the epiotics; and 
NATURE 

[Fune 22,1871 

below, and external to these a bilobate pair, belonging 
largely to the pterotics, but also to the ex-occipitals. 
The flat top of the skull, up to the exit of the fifth 
nerve, is square, the top of the cranium then narrows 
suddenly to half the breadth of the square part. 
On each side, there is, in the broad part of the 
skull, a large overhanging eave, below which is the 
double recess which forms the glenoid cavities for 
the hyo-mandibular. If the large parietals which 
meet at the mid-line were removed, we should see 
the “ great upper fontanelle,” bounded behind by the per- 
fected occipital arch, and laterally by the cranial and 
auditory elements. Indeed, the term cranio-auditory 
elements would be a correct term for several of these 
bones, the auditory capsule coalescing very early with the 
rising crests of the investing mass, and the subsequent 
ossifications enclosing both the sense-capsule and the 
membranous cranium. Behind, the expanded occipital 
region is largely indebted to the “epiotics,” and “pterotics,” 
two pairs of which bones are really primarily related to the 
cartilaginous auditory sac. There is no opisthotic, and 
the large “ pro-otic” is surmounted by a part of the posterior 
sphenoid, which is to be found in the bird, but not in the 
reptile or mammal. I allude to the post-frontal, a great 
outstanding projection from the “ala magna,” a crested, 
fore-turned, supero-lateral element of the primordial skull. 
In front of the “ foramen ovale,” the “ ali sphenoids” wall-in 
the skull; they are unusually large for an osseous fish ; they 
rest upon an inverted “saddle” of bone, with a free fore- 
edge. This is the fish’s basi-sphenoid, and corresponds to 
the pre-pituitary part of the human basi-sphenoid, and to 
its anterior clinoid region. In high-skulled fishes this 
bone is Y-shaped, the descent of its long crus showing 
that the “meso-cephalic flexure” of the embryo is never 
wholly recovered from in these fishes; and its slender 
size showing that the connective band which brought the 
investing mass into union with the “trabeculz cranii,” was a 
feeble strip of cartilage. Behind the saddled-shaped 
basi-sphenoid of the eel is the open pituitary space, 
which, as in birds, is merely closed below by the ossifica- 
tion of sub-mucous fibrous tissue, in the form of the para- 
sphenoid. The large basi-occipital, which encloses all the 
retiring notochord that belongs to the skull, helps to form 
an elegant tri-radiate synchondrosis in the floor of the 
skull; for all that part of the “investing mass” from which 
the notochord had retired, is invested, not by a_basi- 
sphenoidal ossification, but by the huge “ pro-otics” which 
meet at the mid-line, behind the open pituitary space. The 
structures of the skull that have morphological continuity 
with the vertebral column cease behind the optic 
nerves, and even the parts surrounding the pituitary body 
are of a secondary or connective character, bringing 
the true cranial structures into fusion with parts de- 
rived from the first or pre-stomal facial arch, the trabe- 
cule cranil. Now there comes in a most important con- 
dition of the skull of the eel ; forthe anterior sphenoidal 
region has no cartilaginous walls whatever ; the roof is 
formed by the narrow frontals (frontal in the conger) ; the 
side walls are membranous, and the floor is that sub- 
mucous bone the para-sphenoid. In young eels, 5 in. 
long, the trabeculae may be traced to their union with the 
“investing mass” in the pre-pituitary region ; but there 
they unite with each other, and in the anterior sphenoidal 
region, instead of turning upwards to form a skull floor, 
they grow downwards, investing the convex upper face of 
the para-sphenoid (see Fig. B), Over the optic region 
the Azerotics overlap, in the conger they nearly reachas far 
as the hinder end of the bony ethmoid; and here the frontal 
sends out a few post-orbital snags, and sends downwards on 
each side a thick post-orbital process, which articulates 
with the ali-sphenoid. At this part the narrow skull bends 
downwards ina Roman-nosed manner. The solid nasal 
region in front is of equal length with the long membranous 
interorbital space ; these are separated by the large, thick, 
