described in the present work: in Scopelus of the Scopelidae, Supino (’01/02) shows a large myodome,
and I find one in Saurus griseus of the same family: in Crenilabrus pavo, of the Labridae, and in
Trachurus trachurus, of the Carangidae, I find a myodome well and normally developed: in Hoplo-
stethus, of the Berycidae, Supino (’04) shows a normal myodome, and Starks (’04, p. 602) gives, as
one of the characteristics of the Berycoidea, „Myodome large in front, closed abruptly behind, or
open to the exterior posteriorly only through a pore“ : in Fistularia, of the Aulostomi, it is said by
Swinnerton not to be developed, but, as already stated, this may be incorrect : in a Mediterranean
Sargus, and in Chrysophrys aurata, both of the Sparidae, I find a well developed myodome; as I also
do in a Mediterranean Xiphias of the Xiphiidae.
I t is thus seen th a t a myodome is found in certain living representatives of nearly all of the
earliest known families of teleosts; and that certain of these families, certain of the living representatives
of which possess a myodome, are found in earlier geological periods than any of the families
of the Teleostei the living representatives of which are known not to possess it. Certain of the Stylodontidae,
the earliest known representatives of the Lepidosteidae, are also said to possess a myodome.
The palaeontological record, as given by Zittel, thus certainly indicates th a t the conditions from
which the myodome is developed are not to be looked for in either Lepidosteus or Silurus, but in
fishes belonging to earlier deposits than those in which those teleosts and ganoids th a t possess a myodome
are found; and the only fishes so found, living representatives of which are known, are, in
Zittel’s terminology, the Selachii, Dipnoi, Chondrostei, and Crossopterygii.
In th e Selachii, Gegenbaur (’72) shows a thick interorbital wall, and a pitu ita ry fossa (Sattel-
grube) which lies between, or b u t little posterior to, th e posterior portions of the orbits. The canalis
transversus, which transmits the pitu ita ry vein, lies in or b eneath the b ottom of this fossa (Hexanchus,
Mustelus, Galeus), or b u t slightly posterior to it (Heptanchus, Scymnus). In Ceratodus (Bing & Burck-
hardt, ’05, P. 523) a similar interorbital wall and pitu ita ry fossa are found; and the same is tru e of
Acipenser (Parker, ’82a). In Polypterus there is a thick interorbital wall, and Pollard (’92) shows,
in th e cranial cavity of this fish, a cartilaginous shelf which, as already stated, closely resembles the
prootic bridge of Amia. Bing & Burckhardt (’05, p. 571) show this bridge much more inclined than
Pollard shows it, and they show, as Pollard does, what is probably the saccus vasculosus, projecting
backward beneath th e bridge. No mention is made, in either Ceratodus, Acipenser or Polypterus,
of a canalis transversus or pitu ita ry vein, b u t this vein must certainly exist in each of these fishes,
an d must lead into the orbit of either side, as it does in the Selachii and Teleostei. Imagine the orbits,
in either one of these several fishes, to be enlarged and deepened. This would necessarily shorten
th e canal traversed b y the pitu ita ry vein, and would, if sufficiently continued, bring the pitu ita ry
fossa into the hind ends of th e orbits, much as it is actually found in D actylopterus and Gadus. Certain
of th e eye-muscles would then almost necessarily have the ir points of origin transferred to this pituitary
pocket, and a myodome would be established.
This being accepted as the manner of origin of the myodome, do Lepidosteus and the Siluridae
present a primary or a secondary condition? I t is evident th a t they might be considered as presenting
either one or the other, but it seems to me th a t both of them present a primary condition, for, as
already stated, if the anterior edge of the prootic bridge of Polypterus were to be bent downward
until it touched and coalesced, everywhere excepting in the middle line, with the underlying floor
of the cranial cavity, it would give rise to a condition closely resembling th a t found in Lepidosteus;
and if the cross-canal, thus produced, were to be invaded by the surrounding cartilage until only
a slight pit was left on its anterior surface, it would give rise to the conditions found in Ameiurus.
Under this assumption the condition of the myodomic region, as it exists, both in Lepidosteus and
Ameiurus, would be primary and not derived secondarily from a pre-existing myodome. But t his
presupposes, if Ameiurus can be considered as typical of the Siluridae, either th a t the Weberian
apparatus has been developed independently in the Siluridae and the other families of the Ostario-
physi, or that the myodome has been developed, in those families of the Ostariophysi in which it is
found, wholly independently of its development in other teleosts. For that a myodome could have
been developed from the condition found in Ameiurus seems most improbable, Ameiurus quite certainly
representing the end of a line in which the saccular portion of the myodomic region is aborting,
whether it be primarily or secondarily.
Still another suggestion regarding the myodome is th a t its basi occipital extension may have
been developed in relation to a vertebral depression on the anterior surface of th a t bone. In Trigla
lyra, I have shown that, th a t depression in the anterior end of the basioccipital th a t lodges the posterior
portion of the myodome is lined with a layer of dense bone th a t is similar to the bone th a t lines
the vertebral depression in the hind end of the same bone, and th a t these two linings of dense bone
are connected by a small median line of similar bone. This suggests, as I have already stated, that
the myodomic depression on the anterior end of the basioccipital of this fish is, like the depression
on its hind end, a vertebral depression, and if this be true of this fish, it must also be true of all other
fishes in which the myodome has a basioccipital extension. In Gadus and Saurus the myodome
has no basioccipital extension, but in both these fishes there is, nevertheless, a depression on the
anterior end of the basioccipital, and this depression — although lodging no part of the myodome
would certainly seem to be the homologue of the myodomic depression of other fishes. This depression,
in Gadus and Saurus, is continuous with a large vacuity in the hind edge of the prootic, and if the
one is a vertebral depression on the anterior end of the basioccipital the suggestion is evident that
the other might be a depression in some way related to a similar depression on the hind end of the prootic.
The prootic vacuity both of Gadus and of Saurus is in communication, by the intermediation of the
hypophysial fenestra, with the myodomic pocket on the anterior surface of the prootic. The supposition
is thus evident th a t a myodomic pocket might have been first developed in relation to a
vertebral depression on the anterior surface of the prootic region, and th a t this depression, pushing
backward and gradually obliterating or absorbing a similar depression on the posterior surface of the
same region, and then even occupying also a vertebral depression on the anterior end of the basioccipital,
has given rise to all known forms of the myodome. But this necessarily attributes a vertebral
origin to the basioccipital and prootic, for which, in the case of the prootic, there is no apparent
warrant. Furthermore, according to Swinnerton’s descriptions of the development of the basioccipital
in Gasterosteus, even the depression in the anterior end of th a t bone can not be a vertebral one. For
th a t author says (’02, p. 524) that, in th a t fish, a crest of membrane bone grows downward, in the
middle line, from the ventral surface of the primary portion of the basioccipital, and that „within
the substance of the fore part of this crest is a cavity which opens in front and receives the hinder
end of the external rectus muscle; this is the homologue of the anterior conical excavation of the
basioccipital of the Pike and many other teleosts“.