SENSE ORGANS OF AMIURUS. 377 



neural arches of the third and fourth vertebra in the young. (See the 

 horizontal section, Fig. 12, PI. 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, PI. 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, PI. 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, 

 PI. 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 ent over the second vertebra, so its transverse process is obsolete.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 vertebra?. (See Figs. 12 and 13, PI. IV.) We 

 shall meet with a further rudiment of the second neural arch shortly. 



Like the more posterior vertebrae the fifth is amphiccelous ; the 

 posterior cone of the fourth is of large size, (Figs 8 and 13, PI. IV.) 

 while the anterior cone is very small and the intervertebral growth 



