635 
homologue of that wall, as seems so probable, the only possible ex- 
planation of the differing conditions in these fishes would seem to be 
that there is, in fishes, a primarily somewhat independent mass. of 
mesoderm cells lying lateral to the neurocranium and dorsal to the 
dorsal ends of the mandibular and premandibular arches, in the position 
of the pharyngeal elements of the branchial arches, which pharyngeal 
elements are wanting, as independent structures, in the mandibular 
and premandibular arches of all fishes. The tissue represented by 
this mass of cells evidently would be capable of chondrification, and 
to a different extent in different fishes, and when chondrified the 
cartilage so formed might, in certain fishes (and in mammals), be so 
intimately associated with the neurocranium alone as to appear as a 
definite part of that structure, while in other fishes, as in Ceratodus 
(and in amphibians and reptiles), it might be so intimately associated 
with the palatoquadrate as to lead to its being considered as primarily 
a part of that structure, its later fusion with the chondrocranium then 
being considered secondary. Furthermore, it would seem as if the 
supposititious premandibular portion of this mass of cells gave rise to 
the processus ascendens of Ceratodus, and the mandibular portion to 
the processus oticus of that fish, this accounting for the definite and 
constant relations of these two processes to the profundus and trigeminus 
nerves, the nerves, respectively, of the premandibular and mandibular 
arches. Mesoderm cells correspondingly related to the hyoidean arch 
might give rise to some portion of the otic capsule and to its de- 
rivative, the operculum, and probably also to the teleostean hyoman- 
dibular. Whether the selachian eyestalk would, under this interpretation 
of the conditions, arise from the chondrification of the premandibular 
portion of these cells and hence be the homologue of the processus 
ascendens, as I have lately suggested, or be a wholly independent 
structure, needs further investigation. 
The processus oticus thus being represented, when present, in 
Amia, teleosts and selachians, in a part of the cranial wall of those 
fishes, it is evident that the processus metapterygoideus of Amia, 
and its homologue in teleosts and selachians (Aruıs, 1914a), can not 
be a processus oticus, as I and most other authors have heretofore 
considered it; and as the relations of this process, in Amia, teleosts 
and Heptanchus, to the nerves and vessels of the region do not permit 
of its being considered as a processus basalis, it would seem as if it 
must be considered as a special process developed in relation to, or 
