— 191 — 



motorius, a sccoud the root uf the trigeniinus accompanied by a single trunk from which arise the 

 ophthalmicus and buccalis lateralis, and a third the truncus facialis accompanied by the palatinus 

 facialis and ncrvus abducens. The root of the trigeminus evidently includes the radix profundi, 

 bat whether the ganglion of that nerve is intracranial or extracranial is not evident. It is however 

 plainly evident that the cartilaginous floor of the cranium, immediately posterior to the pituitary 

 fossa, is a proötic bridge, and that the side wall of the cranium on either side of the pituitary region 

 represents the inner wall of the trigemino-facialis Chamber, the outer wall of that Chamber, and also 

 the ventral wall of the myodome, being wholly of membrane. 



It is thus evident that no positive conclusion can be formed as to which one of the two floors 

 of this part of the skull of fishes is the primary, and which the secondary one, until it is first known 

 what parts of the membranes here concerned belong to the primordial membranous capsule of the 

 brain and what parts, if any, are developed independentl}^ internal to that capsule, as protective 

 coverings to the brain. The myodome, as it actually exists, is however certainly a space in the dura 

 mater. as that membrane is defined by Sagemehl in his descriptions of Amia, and as both its inner 

 and its outer walls may in large part chondrify or ossify as parts of the cranial wall, the myodome 

 is an intramural and not an intracranial Space. This I have already stated in an earlier work ('97b, 

 p. 10), there speaking of the myodome as an intracranial space, but qualifying this by saying that 

 it is ,,a Space that certainly lies morphologically in, and not internal to, the membranous bounding 

 walls of the primordial skull". 



In the labyrinth region also, both the ventral and the lateral walls of the skull of fishes are 

 partly double, the sacculi of the membranous ear lying between the two ventral walls and the semi- 

 circular canals between the lateral walls. The inner one of these two walls is largely membranous 

 in teleosts and the bony ganoids, but it may partly ossify as the mesial processes of the exoccipitals. 

 In elasmobranchs the wall may be largely of cartilage. 



In most prepared skuUs these two walls of the skull of fishes are not apparent, for their mem- 

 branous portions can only be preserved in careful preparations, and are almost always entirely 

 removed. In serial sections of embryos, also, particularly of early embryos, the membranous portions 

 of these walls are apt not to be recognized as such. Yet the fact that they both exist must always 

 be carefuUy borne in niind, for it, alone, permits of a proper comparison of this region, not only of 

 fishes with one another but of fishes with higher vertebrates. 



In the chondrocranium of embryos of Lacerta agilis the hypophysis lies, according to Gaupp 

 ('00, p. 470), in a hypophysial fenestra bounded laterally by the diverging hind ends of the trabeculae, 

 and posteriorly by a transverse bar of parachordal cartilage, called by Gaupp the crista sellaris. This 

 crista sellaris unites the anterior ends of the parachordal plates, and separates the interparachordal 

 fontanelle from the intertrabecular fontanelle, or hypophysial fenestra; thus having approximately, 

 if not exactly, the position of the posterior clinoid bridge of Parker's descriptions of Lepidosteus. The 

 carotis cerebralis, on either side, traverses, in early stages, the postero-lateral corner of the hypo- 

 physial fenestra, but in later stages it becomes entirely enclosed in cartilage, in an independent carotid 

 foramen. Slightly anterior to the hypophysial fenestra lies the subiculum infundibuli, a V-shapcd 

 cartilaginous plate that rises from the hind edge of the interorbital septum. Anteriorly, this plate 

 of cartilage forms part of the hind border of the optic fenestra. while posteriorly it gives support 

 to the anterior border of the lobus infundibularis. The V-shaped plate of cartilage thus has strikingly 

 the Position of the basisphenoid cartilage of Bcorpaena, and this latter cartilage has been shown 



