DEVELOPMENT OF THE SKULL IN THE URODELES. 191 
trabecule) with the auditory capsules, is largely ossified by continuous (probably 
primarily) prootico-occipital centres ( p7.o, é.0) ; these show division below but not above. 
Below, even in the larger specimen (Pl. XX XVIII. fig. 6), the middle third of the 
capsule is cartilaginous; this cartilage can be traced along inwards and forwards into 
the clinoid band, across, and into the side wall, lengthwise, as far as midway between 
the optic and trigeminal nerves (11, V). 
Above, even in the young (Pl. XX XIX. fig. 2), there is only one bone right and left 
(pr.o, ¢.0); and this extends from the occipital condyles to the otic and ascending pro- 
cesses (0¢. p, @. p), in which it ends as a broad plate, looking forwards and a little outwards. 
The square part which runs into the otic process, and the rounded growth of bone 
which runs into the ascending process, are connected by a narrowish belt of bone which 
forms the fore boundary of a pyriform cavity, partly covered in by a process from the 
outer tract of bone; this cavity, which runs into the auditory capsule, ends behind in a 
sigmoid slit, which turns inwards and then a little outwards, following the outer margin 
of the eminence caused by the anterior canal (a.s.c). It is bounded outside by the 
horizontal canal (. s.¢) in the same manner. This is the “ aqueeductus vestibuli (ag. v),” 
and is to be seen again in Polyodon (see Bridge, Phil. Trans. 1878, pl. lvi.); it is the 
original auditory involution left unclosed in the capsule. 
The auditory capsule sends out a double horn on each side; these projections are 
falcate, and have their convexity looking inwards and backwards, like the parietal 
bones in the Lacertilia. 
These two pairs of projections, the outturned occipital condyles, and the parietal 
bones have all the same contour, and are all parallel with one another ; the pedunculate 
stapes (st) adds a fifth to these postero-lateral wings of the skull, the whole form of 
which, from end to end, is that of a flat, subtriangular axis growing out in all directions 
into spines and leafy growths. 
The stapes (st) is pyriform, and ends in a flat notched lobe, which may be an 
additional part that has coalesced with it, a pharyngohyal. 
The upper and lesser wing of the auditory capsule is the epiotic projection (ep) ; it is 
bound down by the parietal horn and the sharp apex of the squamosal (p, sg); the 
lower, larger, and outer horn is the “ pterotic” projection, a part very large in fishes, 
and well seen in some large American Frogs (e), Rana pipiens, Cystignathus ocellatus. 
Leaving out for the present the roots of the suspensorium (4. p, pd, ot.p), we see a 
remarkable pair of postorbital projections (sp.o) growing in front of the prootic region 
(pr.o), but not ossified by it; they are between the ascending process (a. p) and the otic 
process (ot.p), and may be called the “ preauditory horns.” These growths (as we 
shall see in the larva of Triton cristatus, Pl. XL.) are productions of the investing 
mass or hinder parachordals, and of the trabecular apices ; for these oblique lamine, the 
hinder parachordals, wind round the front of the capsule, and appear in front of its 
