HYOMANDIBULA OF THE GNATHOSTOME FISHES 621 
hypertrophied middle one or ones of the branchial rays of the 
mandibular arch. 
In the Chondrostei, the single dorsal articular head of the hyo- 
mandibula apparently corresponds to the anterior articular head 
(prefacialis portion) of the teleostean hyomandibula, and the 
interhyal to the interhyal of those fishes. The extrabranchial 
of the hyal arch is apparently wholly wanting, but the extra- 
branchial of the mandibular arch may possibly be represented in 
a little process, or a small and independent cartilage, found on 
the external surface of the hyomandibula. The symplectic is 
probably homologous with that of the Teleostei. 
In the Dipneusti (Ceratodus) the hyomandibula, as properly 
identified in recent investigations, is formed by the fusion of an 
interarcual cartilage with the pharyngohyal, and accordingly 
corresponds to the anterior articular head (prefacialis portion) of 
the teleostean hyomandibula. The hyomandibula of Huxley’s 
descriptions of the adult is probably the extrabranchial of the 
hyal arch, and the interhyal of Ridewood’s descriptions the 
epihyal. 
In the mandibular arch, the extrabranchial is found either as 
the independent so-called spiracular cartilage of the Batoidei, as 
the processus oticus palatoquadrati of Ceratodus, or as the post- 
trigeminus portion of the lateral wall of the trigemino-facialis 
chamber of the Holostei and Teleostei. The processus ascen- 
dens palatoquadrati of Ceratodus and the pretrigeminus portion 
of the lateral wallof the trigemino-facialis chamber of the Holostei 
and Teleostei are either the strict serial homologues of the an- 
terior articular head of the hyomandibula of the Holostei and 
Teleostei, and hence interarcual cartilages lying between the 
* mandibular and premandibular arches; or they represent the 
extrabranchial of the premandibular arch here fused with the 
pharyngeal element of the mandibular arch. 
In vertebrates higher than fishes it is evidently possible that 
any one of the several types of hyomandibula found in fishes 
might have been independently developed, or, the line of descent 
of these vertebrates not being known, have been retained by 
inheritance, and the varying relations of the nervus facialis to 
