THE LAMPREY. 5 



case, and unite with one another above the brain in a narrow 

 band, the so-called occipital arch (o.c). The side walls 

 are perforated with apertures for the exit of the optic (II), 

 and of the fifth and seventh (V) nerves. 



5. The auditory capsules (Figs, i and 2, au\ paired 

 ovoidal masses of cartilage, fused with the basal plate and 

 cranial walls, and each containing a cavity for the correspond- 

 ing auditory sac ( 95) ; their inner walls bound laterally 

 the posterior part of the brain cavity, and are pierced with 

 apertures for the auditory nerves. 



6. The nasal capsule (Fig. i, na ; Fig. 3), a concavo- 

 convex plate of cartilage, of irregularly oval form, lying with 

 its concave side forwards, in the posterior wall of the nasal 

 sac ( 89), and forming the anterior boundary of the cerebral 



FIG. 3. Petromyzon marinus. The nasal capsule, front view (nat. 

 size). I, olfactory foramina. 



cavity (see Fig. 4 na.c). Near its centre it is perforated by 

 two oval apertures (Fig. 3, i) for the passage of the 

 olfactory nerves. 



7. The subocular arches, each consisting of an 

 anterior lateral process (Fig. i, a.lp\ springing from 

 the anterior end of the hard palate, and of a posterior 

 lateral process (/././>), arising from the basal plate just 

 beneath the auditory capsule. Both processes pass down- 

 wards, outwards, and forwards, and meet with one another 

 at an acute angle. 



8. The styliform processes (Fig. i, st.p\ cylindrical 

 rods springing one from the posterior edge of each posterior 

 lateral process, near its proximal end, and taking a directly 

 downward course. 



