SKELETON. 73 
In the hyoid and branchial arches ossification occurs to a greater or 
less extent, the resulting cartilage bones having the same names as the 
corresponding cartilages. There are never any membrane bones in 
this region. In the teleosts the hyomandibular ossifies as two bones, a 
dorsal hyomandibular and a lower symplectic which connects with the 
quadrate. There is, however, a considerable amount of union between 
the various arches in the adults of all tetrapoda, where the branchial 
respiration is lost and the arches have to assume other functions than 
the support of gills. os 
The mode of suspension of the K 7 
jaws varies. In a few elasmobranchs L AN a 
the pterygoquadrate articulates di- \ : Mf Me [ 
rectly with the cranium (amphistylic); ; Ct: 
in others it is suspended by ligament | 
and by the interposition of the 
hyomandibular between the otic cap- 
sule and the hinder end of the jaw 
(hyostylic); while in all groups above 
the fishes the pterygoquadrate is more | 
or less completely fused with the N 
cranium (autostylic). 
The ear bones or ossicula audi- 
tus are best treated together here, } 
although their consideration requires — yg. 49 Diagram of the middle 
the mention of structures not yet de- eat of a lizard, after Versluys. a, 
3 * articulare; ¢c, columella; ec, extracolu- 
scribed. The ear bones occur only mella; h, hyoid; ie, inner ear; mpt, 
in the tetrapoda; they present several ee eae tee 
modifications not readily homologized s, stapes; #, tympanic cavity; tm, 
with each other, though they all have Ee eee 
the same function of conveying sound waves across the tympanum 
to the inner ear. In all there is an opening, the fenestra vestibuli 
(f. ovale) in the lateral wall of the otic capsule, which is occu- 
pied by a movable bone, the stapes, of uncertain homologies, but 
probably representing the hyomandibular of the fishes, which otherwise 
is lacking in all tetrapoda. This view is the more probable since in 
some vertebrates the stapes is connected developmentally with the rest 
of the hyoid arch. 
In urodeles and cecilians a slender process extends from the quad- 
rate across the poorly developed tympanic cavity to articulate with the 
