HYOMANDIBULA OF THE GNATHOSTOME FISHES 597 
third and fourth arches and accordingly had the position of 
an interarcual cartilage rather than that of a pharyngobranchial. 
The nervus hyomandibularis facialis and the vena jugularis, 
in embryos of Ceratodus, always issue through the posterior op- 
ening of the trigemino-facialis chamber (Allis 714 ¢). The wide 
space, in the adult, between the two foramina for the nerve and 
vein, to which reference has just above been made, then indicates the 
extent to which the posterior portion of that chamber has been 
enlarged by the invading and enveloping growth of the cartilage 
of the region, and that edge of the suspensorium to which the 
hyomandibula is attached still represents the hind edge of the 
lateral wall of the chamber. The so-called hyomandibula thus 
lies not only at a considerable distance from the neurocranium 
but also definitely lateral to the vena jugularis, and hence not at 
all in the relation to that vem and the neurocranium which both 
the pharyngohyal and epihyal have in the Elasmobranchii. 
In embryos of Ceratodus the cartilages in the dorsal half of 
the hyal arch have been described by Sewertzoff, Firbringer, 
Krawetz, Edgeworth and Greil, and not only do the homologies 
proposed by these several authors for the several cartilages here 
developed differ greatly, but even the descriptions themselves 
differ, in certain respects, markedly from each other. 
Sewertzoff (02) found, in an embryo of Ceratodus somewhat 
_ older than Semon’s Stage 47, a little cartilage which is said to 
lie immediately posterior to the nervus hyomandibularis facialis 
and to connect the ceratohyal with both the processus oticus pala- 
toquadrati and the auditory capsule (mit dem Pr. oticus resp. 
mit der Ohrkapsel). The ventral end of this little cartilage is 
said to be attached to a ligament which is evidently the hyosus- 
pensorial ligament of descriptions of the adult, while its upper 
end simply touches, without being attached to, the processus 
oticus palatoquadrati dorsal to the point of attachment of the 
ligament and dorsal also to the nervus hyomandibularis facialis. 
Sewertzoff considers this cartilage to be a rudimentary hyoman- 
dibula and the homologue of the cartilage so-named by Huxley 
in the adult. He did not find this cartilage inembryos of Stage 47, 
or younger. To facilitate the descriptions and avoid confusion 
it will hereafter be referred to, in brackets, as the cartilage Ex. 
