— 190 — 



oi it, the canal for the pituitary vein leads on either side into the orbit, lying ventral and then lateral, 

 or antero-lateral to the hypophvsis. 



The conditions in Lepidosteus thus all indicate that the nayodome of Amia would arise if certain 

 of the recti muscles of the former should force an entrance into the cranial cavity, and then into 

 the saccular space, traversing, in this process, the foramen and then the canal for the pituitary vein, 

 The recti muscles, as already stated, already have their origins in the immediate vicinity of the foramen 

 for the vein, and one head of the rectus externus has, in my 80 mm specimen, already acquired a very 

 considerably intracranial extension. If this head of this muscle were to continue its progress, at the 

 same time increasing largely in size, it would, following the vein, make its way backward between 

 the cartilaginous floor of the cranial cavity and the overlying membrane, would force that membrane 

 and the overlying brain upward in the cranial cavity, and might finally reach and break down the 

 anterior wall of the cross-canal; the myodome of Amia thus being produced. And this is, in principle, 

 the manner in which Sagemehl suggests that the teleostean myodome did arise, although in the details 

 of his explanation he is wrong. The conditions in Lepidosteus might however represent a stage 

 in the abortion of the conditions that preceded and led to the establishment of a myodome, those 

 pre-existing conditions being represented in Polypterus; and this seems to me the proper Interpretation 

 of the facts, as will be later discussed. 



But, whatever the origin of the myodome, there are in Lepidosteus, as there are in Amia and 

 Scorpaena, two floors in the pituitary region of the cranial cavity, the dorsal one of these two floors 

 being membranous to a diSerent extent in each of these three fishes. The lateral walls of this part 

 of the skull are also double in each of these three fishes, one or the other of the two walls being 

 also membranous to a different extent in each of the fishes. In the space between the two floors 

 lies the myodome, either actual or potential, while in the space between the two lateral walls lies 

 the trigemino-facialis chamber. In Dactylopterus the conditions are modified and obscured by 

 the fact that the dorsal floor arises from or near the anterior edge of the ventral floor and is inclined 

 at a marked angle to it. 



According to Starks ('05), the ventral floor of the myodome of fishes ,,is the true cranial base", 

 the dorsal floor being simply a septum of secondary development; and the conditions that I have 

 described in Amia would seem to favor this Interpretation, the dorsal wall of the myodome chondri- 

 fying, in that fish, much later than the ventral wall. But this condition is not invariable in fishes, 

 as Handrick's ('Ol) descriptions of Argyropelecus show. In that fish the myodome is said to begin 

 beneath the membranous pituitary fossa, and from there to extend backward immediately beneath 

 the cartilaginous floor of the cranium until it reaches and ends against the ,, anterior outer wall" of 

 the buUa that encloses the sacculi and lagenae. The side walls and floor of this myodome are not 

 particularly described, but they are evidently formed by portions of the hard, modified, connective- 

 tissue membrane that is said to largely cover the roof and side walls of the cartilaginous cranium, 

 and the trigemino-facialis chamber of either side would seem to be in direct communication with the 

 myodome, as it is in Amia. The trigeminus ganglion is said to be an extracranial one, and to lie 

 in the upper corner of the myodome. The ciliary ganglion is also said to lie in the myodome, while the 

 sympathetic ganglion is said to lie on the outside of the side wall of the myodome, and the communis 

 and lateralis ganglia of the V — VII complex, which evidently form the large so-called facialis ganglion, 

 to lie entirely within the cartilaginous cranium. The cartilaginous wall of the cranium is said to 

 be perforated, on either side, by three separate foramina, one of which transmits the nervus oculo- 



