59 
its lower arm containing “einen Kochenkanal, der an der inneren Fläche 
des breiten Mittelstückes frei und nach oben gerichtet sich öffnet”. 
A wholly separate and apparently purely dermal bone is found asso- 
ciated with it, in both Gymnarchus and Mormyrus. It is called by 
ErpL the “Gehördeckel, den man vielleicht mit den äußeren Gehör- 
gangknochen mancher Säugetiere, z. B. des Bibers, vergleichen könnte”. 
This semicircular bone, traversed in part by a canal, and covered by 
a separate bone, thus strongly recalls the circular canal of the Stego- 
cephali, the “Gehördeckel” of Gymnarchus occupying, approximately, 
the place of the supratemporal bone of the Stegocephali. With other 
Teleosts the conditions described in Gymnarchus seem to offer no basis 
whatever for a direct comparison. 
In Protopterus, the squamosal of WIEDERSHEIM’s descriptions 
(No. 34) seems to be the homologue of Gaupp’s paraquadratum, and 
hence, if it be such, it can not, according to Gaupp’s conclusions, be 
the homologue of the squamosal of Ganoids and Teleosts. In Cera- 
todus, and in fossil Dipnoids, a bone is found (No. 15, Bd. II) that 
seems to more closely resemble the squamosal of other fishes. As, in 
both Protopterus and Ceratodus, lateral canals exist, and are said to 
traverse the squamosal region of the skull, bony substance strictly 
homologous with the canal component of the bones of Teleosts and 
Ganoids must exist in these fishes also. The canals, however, are 
nowhere sufficiently described or figured to allow of any definite 
opinion regarding the bones that are related to them. A separate, 
bony, otic capsule is described by FRITSCH in Ctenodus. 
We thus see that the squamosal of fishes is composed of a canal 
component and a deeper-lying component which may be either a so- 
called membrane bone, or such a bone fused with a so-called primary 
ossification. The primary ossification may be wholly wanting, and 
perhaps the canal component also. Furthermore the canal component 
may be found entirely separate from the underlying bone, may be found 
fused simply with an underlying membrane component, or may be found 
fused with such a component and with a so-called primary ossification, 
which latter ossification, alone, is traversed by the external semi- 
circular canal. The canal component, apparently always united with 
the underlying membrane bone, may, as a so-called dermal bone, be 
found fused with other, adjoining, dermal bones; while the primary 
ossification may be {fused with other adjoining primary ossifications 
(Ctenodus), or with such ossifications and the intercalar (Polypterus). 
It is the primary part of the bone, and not its “Deckknochenanteil“, 
that gives articulation to the hyomandibular. 
