OF THE SKULL IK LEPIDOSTEUS OSSEUS. 
465 
little behind its upper part, and quite behind the auditory capsules. Here the 9th and 
10th nerves and their ganglia (IX., X.) are laid bare, and the heart (h.) is cut through 
at its thickest part. The investing mass has become narrow and hour-glass shaped 
in section; it sends upwards a cartilage which thins out gradually upwards ; this is 
the exoccipital (e.o.), which becomes supraoccipital above. The exoccipital bones 
are forming as embracing ectostoses; the notochord has its sheath, the rudiment of 
the basioccipital (b.o.), and under the investing mass the parasphenoidal forks are 
seen. Here there is one pharyngo-branchial ( p.br .) and the distal parts of two or 
three arches, with the basal piece ( b.br .) lying over the heart (h.). 
Section 13.—This last section (Plate 33, fig. 13) is close in front of the first vertebra ; 
it catches the last pharyngo-branchial (for h.br. read p.br.), and shows some pharyngeal 
teeth. Only half the arch comes in here, but it shows well the outer and inner laminae 
of the exoccipital bone (e.o.). Here the notochordal bony sheath (nc., b.o.) is thicker, 
and the splintery ends of the parasphenoid narrower than in the last section. 
In the last three sections bony laminae are forming beneath the skin ; these and 
their relations will be better understood by reference to the figures and descriptions of 
more advanced stages. 
Fifth Stage.—Young Lepidostei 2 to 2j inches long. 
In this stage the skull is rapidly acquiring its permanent character (Plate 34, 
figs. 1, 2); the rostral part is now twice as long as the cranial cavity; in the last 
the pre-cranial part was not quite so long as the cranial. 
This is mainly due to the growth of the intertrabecula ( i.tr.) } which already is 
considerably more than twice as long as the very long cornua ( c.tr .). 
The endocranium is now in this specimen (which is 2 inches long) nearly as perfect 
as it will be; the upper fontanelle (fo.) is a short ellipse, with the long diameter axial; 
it is relatively much less, through the growth of the tegmen cranii ( t.cr .), fore and aft. 
The whole endocranium may be said to be pyriform, with an extremely long stalk ; 
the ££ nose ” of the pear is represented by the basioccipital (b.o.). The occipito- 
auditory region is semicircular above, with an apiculation over the foramen magnum ; 
the orbital region is suddenly narrowed, and this gently lessens into the ethmoidal, 
which as gently becomes rostral, and the rostrum slowly lessens to its fore end, where 
it has two small wings. Near its root, however, it has two large wings—these are 
the cornua trabeculae (c.tr.). The floor of the skull is still largely open (fig. 3), and 
the sides are occupied by the large orbital fenestrse (os.f.). The sheath of the noto¬ 
chord (nc.) is most completely ossified below; above (fig. 3), it is still membranous 
in front; the bony sheath is the basioccipital (b.o.); it runs a little, right and left, into 
the basal cartilage. 
The auditory capsules are completely confluent with the chondrocranium ; and their 
canals (a.s.c., h.s.c., p.s.c.) are to be seen through the cartilaginous wall. Above, the 
even form of the skull is not much altered by the canals within; below, the suceulus 
3 o 2 
