OF THE MARSIPOBRANCH FISHES. 
423 
anterior and posterior semicircular canals are short, wide tubes, and the horizontal 
canal is arrested. The meatus auditorus internus (Plate 18, fig. 5, VIII.) is scarcely 
larger than the optic fenestra (II.), so that, for a Fish, the capsule is very perfect 
inside; the space for the entrance of the auditory nerve is much more vertical than 
in the Myxinoids (Plates 10 and 17). The long axis of these elliptical boxes is 
accurately parallel with the axis of the skull; above, they stand out well from the 
skull, but below we see a bridge of cartilage, derived from the parachordal (iv.), 
thrown across the interspace between the capsule and the cranial floor. This scooped 
thin edge of the parachordal, which has become confluent with the capsule, is very 
distinct, for a time, in Tadpoles (“ Batrachian Skull,” Part III., var. loc.) ; it is 
continuous with the pedicle ( pd .) in front, and in the Myxinoids (Plates 9, 10, 16, 
and 17) with the uppermost part of the hyoid arch. 
That the nasal capsule (Plate 18, figs. 1, 3, 5, na.) is essentially double, like the 
trabecular cornu ( c.tr .) is evident; it is a very exact likeness of the dilated crop of a 
Pigeon; its halves are a little less than the auditory capsules ; they are composed of 
soft cartilage. Behind the aperture ( e.n .) there is an emargination, followed by a widish, 
shallow groove ; behind, the median cartilage is somewhat produced as a lobe. The 
cartilage runs three-fourths of the way round (Plate 21, figs. 3-5) up to the posterior 
nasal canal, and only forms a perfect floor behind; the aperture (e.n.) is membranous. 
These things will be best explained by the transverse sections (Plate 21). The roof is 
pierced by vessels, near the middle, at its hinder third (Plate 18, fig. 3, na.) ; in that 
figure the olfactory nerves (I.) must be supposed to be seen through the fontanelle (fo.). 
Craniofacial skeleton of newly-metamorphosed young of Petromyzon marinus. 
(5 inches long). 
These were about half the length of the specimen whose skeleton has just been 
described; and, in development, an intermediate one between that stage and the one 
next to be described. 
Compared with the larger specimens, we see that the cranial notochord is longer, 
being half as long as the cranium proper, so that the para- and pro-chordal regions 
are equal in length. The notochord (Plate 19, figs. 1—3, nc.) projects somewhat into 
the oval pituitary space (b.cf), which is twice as large as in the larger specimens. 
The inner edge of the parachordals is soft both above and below; the two are united 
for a short space below. The outer edges, confluent with the auditory capsules (au.) 
are of less extent, there being a considerable notch fore and aft under the outgoing 
trigeminal, facial, and vagus nerves (V., VII., X.). The hard cartilage is continued 
back, as far as the hinder outline of the auditory capsules, and is then replaced by 
narrowed bands of the soft kind, further than in the last stage. Behind, the pro- 
chordals (tr.) are narrower than the para-chordals (iv.), but they broaden out 
forwards; their larval form is traceable in the upper view (compare Plate 19, figs. 
3 I 2 
