OF THE SKULL IK LEPIDOSTEUS OSSEUS. 
473 
hemispheres (C la .), and is behind the quadrate bone; the other parts are similar to 
those in the last two sections. 
Section 13.—-Here (Plate 35, fig. 10) the hemispheres (C la .) are wider, and with them 
the cranial cavity, whose walls are thinner, and laterally are partly ossified ; these bones 
right and left, are the lateral ethmoids ( l.eth .); they were not seen in the dissection of 
the lesser specimen of this stage ; (2 inches long, Plate 34 ; the one sectioned was 
2\ inches); but these bones are figured in the dissection of the next stage (Plate 38, 
figs. 2, 3). 
Looking at the base of the cranial axis in this and the last three or four sections, 
we see that the parasphenoid ( pa.s.) fits to a cartilaginous mass having a trilobate 
outline below ; this arises from the fact that it was formed by the coalescence of three 
cartilages, viz. : the trabeculse and the intertrabecula. 
Section 14.—The hemispheres in this section (Plate 35, fig. 11, C la .) are rapidly 
widening, and the cranial walls are now deficient, the sides being membranous—the 
orbito-sphenoidal fenestra—in the lower half. The tegmen cranii ( t.cr .) runs down 
the sides half way, and is grooved above ; the lower edge of the cartilage just touches 
the optic nerve (II.). Here the base of the skull has lost its height; it is concave 
above, and scoo23ed below. Here is the hinder end of the intertrabecula, and the 
trabeculae {tr.) are each of them crested below. The suspensorium is very flat here, 
especially towards the top ; it thickens out again above, and is surmounted there by 
a small extraneous bony plate—the mesopterygoid ( ms.pg., see also Plate 34, figs. 1, 2); 
here the pterygoid (pg.) has become a thin plate. The essentially double nature of 
the basi-hyal ( b.hy .) is clearly seen in this section. 
Section 15.—The hinder part 0 f the hemispheres (Plate 35, fig. 12, C la .) are now cut 
through, and the tegmen cranii {t.cr.) is now a thin, sinuous awning thrown over the 
brain-cavity; it is hollow above, and convex at the sides. The frontals ( f ) are becoming 
thin, and the hinder superorbitals {s.ob.) are thick and large. The trabeculae {tr.) 
only are seen in this section, which is through the fore part of the long pituitary space 
(see Plate 34, fig. 3, py. ); they are oval in section, and a space equal to their width is 
filled up between them by the parasphenoid {pa.s.). 
The suspensorium ( q.c.) has thickened again, and still the same bones are applied 
to it, namely: the mesopterygoid, pterygoid, and preopercular {ms.pg., pg.p., p.op.). 
Below, the section was made behind the basi-byal, and through the first basi- and 
hyjjo-branchials ( b.br., h.br .); outside, we see the cerato-hyal {c.hy.), with its bony 
sheath, cut through. 
Section 16.—In this section (Plate 36, fig. 1) the skull is cut through close in front of 
the basi-pterygoids (Plate 34, figs. 1-3, b.pg.) ; in this specimen there must have been 
some little projection backwards from the front tegmen {t.cr.), not seen in the one 
dissected ; this would have made the fontanelle heart-shaped, instead of circular.* 
* This projection from the front “ tegmen ” was seen in the last stage (Plate 32, fig. 4, t.cr.), where the 
fontanelle has a similar shape to that of a young Salmon of the 2nd week. (“ Salmon’s Skull,” Plate 4, 
figs. 1, 2.) 
3 P 2 
