ZOOLOGY. 199 © 
stapes. In existing reptiles, it is only proximally expanded, and 
distally a slender rod terminating in the cartilaginous expansions 
called by Huxley* suprastapedial and extrastapedial; the latter 
being, as the same author shows, the support of the stylohyoid and 
other elements of the hyoidean arch, and with the suprastapedial 
the homologue.of the incus. The expanded distal end of the bone 
marked stap, in Ichthyosaurus, looks as though it were the homo- 
logue of the cartilaginous expansions mentioned, in which case that 
bone becomes stapes and incus combined. This seems to us very 
probable.t 
As a whole, this bone is in that case homologous with the hyo- 
mandibular of the Sharks and Teleostei. This has been pointed out 
by Huxley on embryological grounds to be the case with the incus. 
If the element (stap) in Ichthyosaurus represent both stapes and 
incus, the same is probably true of the hyomandibular. 
Turning again to the squamosal, we find it appears to possess an 
extraordinary development. Besides forming the posterior portion 
of the zygomatic arch, as in other vertebrata, and forming part of 
the combination which supports the quadrate, as in Reptiles and 
Batrachia generally, it sends down 
behind the quadrate a plate for 
more than one-third the length of 
the latter to the superior margin 
of the stapes. Instead of joining 
the parietal or opisthotic at its pos- 
terior margin, it is continued in- 
wards to near the apex of the 
supraoccipital, and bending for- 
wards continues, in company with 
its fellow of the opposite side on F 
the inner face of the temporal fossa, to a point above the middle of 
the orbit, where it unites suturally with what may be called the pari- 
* In a most valuable essay on “The Representatives of the Malleus and the 
Incus of the Mammalia” (Proceed. Zodl. Soc., 1869, p. 391). : 
t In the serpent Xenopeltis unicolor, a superior process of the stapes (suprasta- 
pedial) is ossified, and a separate element at the end of the bone (extrastapedial 
vy. stylohyal ?) is also ossified. (See fig. 2.) 
} Fig. 2.—Ichthyosaurus ; cranium; posterior view. Lettering the same as 
in fig. 1, with the following additions : — . 
B. O... Basioccipital. Op. O. . Opisthotic. 
Ex. O. . Exoccipital. Stap. . . Suprastapedial, or hyomandibu- 
Sup. O. Supraoccipital. lar. 
