1869.] THE MALLEUS AND THE INCUS OF THE MAMMALIA. 405 
passes outwards, and is inserted into the upper aspect of that knee- 
like process of the malleus which is fixed to the tympanic membrane. 
The cartilaginous “ styloid’’ end of the hyoidean areh is fixed into 
the wall of the outer and posterior end of the tympanic cavity, very 
near the éneus and stapes; but I can find neither a stapedius muscle, 
nor any ligament representing it. 
It will be observed that the proximal end of the skeleton of the 
first visceral arch (whether it be osseous, cartilaginous, or fibrous), 
like that of the second, remains attached to one and the same part of 
the skull, viz. the outer and upper wall of the periotic mass, external 
to the vestibular sac, throughout the Mammalia and the Sauropsida. 
In Mammals the proximal skeletal elements of the arches (malleus 
and incus) are very generally equal, or the incws may be the smaller, 
In the Sauropsida, the suprastapedial (=incus) is always smaller 
than the quadratum (=malleus). 
In Teleostean and Ganoid fishes, and in the Sharks, the general 
relations of the two arches remain unchanged, but their proportions 
are reversed. 
The only vertebrated animals in which a portion of the first visceral 
cleft remains open throughout life are some Ganoidei and most 
Elasmobranchii, in which, according to Wyman’s observations, the 
spiracle is the remains of that cleft. It follows that any skeletal part 
which bounds the spiracle posteriorly cannot belong to the first, 
or mandibular, visceral arch, but must appertain to the second, or 
hyoidean arch. Now the suspensorial cartilage of the Elasmobranchs 
occupies this position. Its proximal end is attached to the outer wall 
of the auditory capsule ; its distal end bears the proper hyoidean arch. 
Thus it answers exactly to the upper end of the second cartilaginous 
visceral arch, and therefore must contain the homologue of the zzeus. 
But the suspensorial cartilage of the Elasmobranchs is undoubtedly 
the homologue of the hyomandibular bone and symplectic of the 
osseous Ganoidei and of the Teleostei—which, therefore, must, in part 
or wholly, answer to the incus. Where, then, is the homologue of 
the proximal end of the skeleton of the first visceral arch of the fish, if 
the hyomandibular belongs altogether to the second? I find it in that 
prolongation of the quadrate cartilage of the Teleostean which ascends 
in front of the hyomandibular, and is at first quite free from it, but 
afterwards becomes surrounded and replaced by the metapterygoid, 
which eventually helps to bind it to the hyomandibular. 
Thus the puzzling division between the mandibular and the 
hyoidean parts of the suspensorial apparatus in a fish becomes intel- 
ligible as the result of their primarily separate development. 
In the osseous fishes the proximal end of the mandibular arch is 
arrested in its development and loses its direct connexion with the 
skull; but in the Sharks the ascending portion of the quadrate 
atrophies altogether, or is represented merely by pre-spiracular car- 
tilages ; and the quadrate itself forms only the posterior termination 
of the palato-quadrate arch, or so-called upper jaw. 
Proc. Zoou. Soc.—1869, No. XXVII. 
