DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 111 
I may have missed this peculiar structure in some of my dissections ; this is the 
last skull at which I have worked—if so this character will not easily escape me 
again. It appears to me to be a new and rare thing, and one of many remarkable 
cases of the conformity of the Mammalian skull with what is to be found in the 
Oviparous types—high and low. 
Afterwards, I have no doubt, the bony matter transforms still more cartilage, that 
bone soon becoming anchylosed with the ossified tracts of the rest of the lateral 
ethmoids. Whether “posterior paired vomers ” appear or not I cannot say, possibly 
the hindmost lobes or wings of the main bone become distinct for a time, and then 
unite with the bone forming within, so as to complete the compound desmognathisni of 
the Mammalian upper face. 
The anterior paired vomers (v’.) are very small in this type ; they are club-shaped, 
convex below and towards the septum, and hollow towards JAcopson’s organs, of 
which they are the bony correlates. They lie right and left, a little in front of the 
main vomer. 
The skull proper is composed, now, of about equal] parts of unossified and ossified 
cartilage. The anterior sphenoid shows no basal piece below ; the whole of the thick 
beam, from the basisphenoid (Plate 15, fig. 3, p.s., p.e, sn.) to the end of the 
snout, being cartilaginous. 
That beam shows lateral wings where it passes into the alisphenoids, but for the rest 
of its extent, as seen from below, it is simply convex, lessening forwards (Plate 15, 
figs. 1 and 3, p.s., s.7.). 
In this large embryo the orbitosphenoids (0.s.) have lost their ethmoidal and supra- 
auditory attachments, but the wide outer half is still unossified. Their rounded fore 
edge comes short of the retiring margin of the lateral ethmoids, only membrane 
flooring the front part of the cranial cavity there. The ear-shaped upper part of each 
orbitosphenoid runs backwards inside the alisphenoids (fig. 2), but the two broad 
wings of bone at the proximal part are in front of the alisphenoids, and these form 
the front boundary of the sphenoidal fissure, through which the 1st and 2nd branches 
of the 5th nerve (V'*.) escape. The orbitosphenoidal bones are thick, below, fore 
and aft of the optic foramen (II.), and the front thickening nearly touches the vomerine 
fork on one side—the right in the figure—and overlaps it on the left, the concave 
margin of the bone fitting against the rounded inner lobe of the great nasal capsule. 
The hinder thickening of the bone, below, rests upon the ossified angle of the proximal 
part of the alisphenoid (ad.s.). 
The posterior sphenoid is about equal now to the anterior, which it imbricates, 
behind, clearing itself well, outwards, from the rest of the skull wall. 
The bony basisphenoid (b.s.) occupies about three-fourths of its own territory ; 
the whole of this subpituitary part is thick and convex, showing no sign of the 
primary deficiency in the cartilaginous floor, The sides (al.s.) have united, already, 
with their keystone piece, but a groove below marks the old line of division 
