SENSE ORGANS OF AMIURUS. Ouik 
neural arches of the third and fourth vertebra in the young. (See the 
horizontal section, Fig. 12, Pl. IV.) The tube is perforated near its 
posterior margin by the roots of the fifth nerve, not far from its 
anterior margin by the roots of the fourth, and its anterior margin 
has two notches which correspond to the roots of the third nerve. 
That part of the tube which intervenes between the third and 
fourth nerves represents, therefore, the third neural arch ; that 
between the fourth and fifth nerves the fourth neural arch. The 
second neural arch, apart from its cartilaginous rudiment, is entirely 
membranous in the adult. Immediately behind the point of emer- 
gence of the fourth nerve the tube expands into the fourth transverse 
process, which forms a broad, flat plate (Fig. 7, Pl. IV.), extending 
back as far as the fifth transverse process, and forwards to articulate 
by its thick anterior margin with the transverse process of the supra- 
clavicle. Immediately in front of and below the same point the 
modified third transverse process, or ‘ malleus,’ is articulated move- 
ably to the line of junction of the neural canal and the vertebral cen- 
tra. The form of the malleus may be gathered from Fig. 7 ; its 
posterior sickle-shaped end, which rests partly on the ventral surface 
of the fourth transverse process and on the side of the body of the 
fourth vertebra, is really developed in the tunica externa of the air- 
bladder, and its junction with the anterior part is only secondary. 
Fig. 12, Pl. IV. Its anterior part passes forwards and outwards, 
lying in a horizontal plane, and its tip projects slightly beyond the 
anterior surface of the body of the first vertebra. 
The neural tube is continued above into two neural spines. (Fig. 8, 
Pl. IV.) One of these, which projects upwards and backwards from 
over the fourth vertebra, is the fourth neural spine. The other projects 
forwards from over the third vertebra, and is continued as a perichond- 
drial ossification of the cartilaginous roof of the most anterior part of 
the neural canal and articulates in front with the supra- and exoccipi- 
tals above the foramen magnum. As the osseous neural canal is defici- 
ent over the second vertebra, so its transverse process is obsolete. 
The cartilaginous neural arch in the young is, however, quite as dis- 
tinct as in the other vertebre. (See Figs. 12and13, Pl. IV.) We 
shall meet with a further rudiment of the second neural arch shortly. 
Like the more posterior vertebrae the fifth is amphicclous ; the 
posterior cone of the fourth is of large size, (Figs 8 and 13, Pl. IV.) 
while the anterior cone is very small and the intervertebral growth 
