30 MR. W. K. PARKER ON THE STRUCTURE AND 
a small round hole, a large face above for the incudal facet, and a thick dilated base ; 
the neck, with the stapedius muscle (st.m,), and interhyal (z.hy.) attached to it, is short. 
Two changes have appeared in the arch of the hyoid; one is the complete fusion of 
the epihyal with the auditory capsule (see, also, Plate 2, figs, 4 and 8, e.hy.); and 
the other is the degeneration of the lower sub-segmented part of the epihyal 
into a ligamentous tract. The fused upper part has thickened; over it the chorda 
tympani (VII'.) passes from the facial nerve (VII.), behind. 
The ceratohyal (chy.) has a larger upper, and a shorter lower, segment; then 
comes the thick ovoidal hypohyal (h.hy.), articulated to the trifoliate basihyo- 
branchial (b.4.br.) ; the median lobe, here, is a rudiment of the long basibranchial 
series of the Ichthyopsida ; the lateral processes are the ‘“ thyrohyals ” (¢.Ay.). 
The annulus tympanicus (Plate 5, fig. 4, a.ty.) 1s forming out of a ligulate, 
U-shaped tract of softish cartilage, 
Endocranium of Tatusia hybrida.—Third Stage (embryo, 3 inches long). 
Although this embryo is one-third longer than the last, it has altered but little 
except in the size of the parts; yet some new osseous centres have appeared, and the 
others are more perfect. The endocranium, as seen from above (Plate 5, fig. 1), is a 
remarkable and a very complex structure. The basal line is occupied for the front 
two-thirds, by the huge nasal labyrinth; a large space, right and left of the remaining 
third, is taken up by the organs of hearing; the eye-balls occupy a considerable space, 
but they have dominated the cranial growths but little ; the sockets are very imper- 
fectly marked out. 
The nasal labyrinth, with its right and left galleries confluent with the great 
median middle wall, is naturally divisible into three regions—the alinasal, aliseptal, 
and aliethmoidal (fig. 1, a/.n., al.sp., al.e.). The first two of these regions are supplied 
with the ophthalmic (orbitonasal) branch of the 5th nerve; the hinder region with 
the olfactory or 1st nerve. So that, although these parts run into each other, there is 
an important difference between them; of course the alinasal region is merely 
vestibular. 
The alinasal region (aé.7.) is but little seen from above (compare figs. 1 and 5); 
there it runs across in front of the symmetrical, imperfect cylinders of the aliseptal 
region (a/.sp.) as a narrow arched band, The last fourth of the long convex roofs of 
the labyrinth belong to the aliethmoidal region (al.c.), and have the inner part of the 
upper turbinals (w.tb.) growing within, and from them. Where this region begins, 
there the whole capsule spreads out, suddenly, into two widely divaricating, ovoidal 
masses, marked with the backs of the bifurcating turbinal outgrowths. 
A large valley, in shape like a Butterfly, with its head backwards, lies between 
the large upper turbinal masses; the “ body” between the “ wings” is the top of the 
perpendicular ethmoid ().¢.), whose highest, crested part is the cartilaginous crista 
