DEVELOPMENT OE THE SKULL IN THE BATEACHIA. 
633 
outturned cornua trabeculae (continuous with the labial, c.tr., u.l.) partly enclose the 
sacs by a tooth-like process ; behind, on the front of the “ palato-trabecular lamina ” 
(ant-orbital), there is a similar tooth. Outside these teeth-like flaps is the internal nostril 
(i,n.) ; it is bounded within by these projections, and the nasal sac in front by the curving 
“cornu;” behind by the pterygo-palatine (p-pg-) ; and externally by the apex of the 
cornu, where it has coalesced with the quadrate, by the intermedium of the chondrified 
prenarial ligament. Below (fig. 2), it is easy to see that the upper labial (u.l.) is largely 
hidden by the underlying trabecular cornua ; above, the reverse is the case, the labial 
tape being stitched on to the upper surface of the shelving frontal region of the trabeculae. 
The conjugational plate ( ppg •) lies on a lower level than much of the trabecular band, 
above (fig. 1) ; it is, indeed, hollow above, in front of the fenestra ( so.j '.). 
Also on a lower plane lie the ventral ends of the first arch, the mandibular, whose 
broad pedicle ( pd .) runs into the trabecula behind its concave lower face (fig. 2). The 
notch between the first and second bar is rounded, and then, both having their conti- 
guous edges convex, they come into contact for some distance, widening to form the 
fenestra (so.f.). All the landmarks round the pedicle are faintly visible, the intense 
chondrification having united this bar to the trabecula, the auditory capsule, and to the 
huge “ tegmen ” ( t.ty .). Where the mandibular pier escapes, on the one hand from the 
tegmen and on the other from the trabecula, it runs as a broadish, straight band up to 
the hyoidean and “ orbitar ” expansions ; the latter of these ( or.p .) is less curved over 
the temporal muscle. From the quadrate condyle ( q .), inwards, the common conjuga- 
tional plate (p.n.l., p.pg.) is of great breadth. 
The free mandibles ( ar ., mk.) are fast thickening, they are more arched, and the inferior 
labials ( l.l .) are shown in their obliquity, dipping slightly into the gently convex chin. 
The free hyoid arch (Plate 57. fig. 2, c.hy., and Plate 58. fig. 1, c.hy., b.hy.) is a solid, 
oblongo-arcuate bar, with ridges and sulci externally, for muscular attachment. The 
basihyal does not segment itself from the basibranchial ( b.hy ., b.br.) ; they form one piece. 
The branchial arches (Plate 58. fig. l.br. 1-4) are a very remarkable structure ; they 
are quite confluent with each other, opening freely within and without by three clefts. 
The first arch is a large bag of cartilage with thin, sinuous walls, through which are 
seen the radiating rows of lophobranchiate branchiae. The second and third arches 
appear outside, as far as the clefts go, as moderately wide bars, but the fourth arch is 
a pouch, much less, however, than the first, and having its external face concave. 
Through the clefts can be seen the interdigitating hillocks of tissue from which the 
dendritic gills grow (see “ Frog’s Skull,” p. 156). 
On the edge of the upper lip a row of about twenty small teeth is seen, but the par- 
ostoses that are to form their foundation are not yet developed. 
The auditory capsule is still unossified, the two sacs are more parallel than in the 
former stage, and the floor of the sacculus coming nearer to the outer edge, the now 
well-formed fenestra ovalis ( f.so .) is almost vertical, on the outer wall, under the tegmen 
tympani. An oval, externally convex cartilaginous stapes (st.) is now formed. The 
MDCCCLXXVI. 4 T 
