OF THE SKULL IN THE COMMON SNAKE. 
403 
Tlie bony matter grows off the cartilage, both laterally and behind, into the con- 
tiguous membrane ; the carotids are en-ringed completely by bone ; this is the foremost 
bone of the cranium, proper, at this stage, for the orbito-sphenoids (os.) are still soft, and 
no presphenoid ever appears. 
The alisplienoidal cartilages ( al.s .) have coalesced with the basal part, and also with 
the auditory capsules ; they are ossifying, and the bone is both notched and perforated 
by the trigennial nerve (5). 
The main periotic centre, the prootic (pro.), is now enclosing the fore part of the 
capsule ; the other two are not visible. These will soon appear. 
The frontals and parietals ( f , p.) are now just gaining the top of the head, and the 
parasphenoid ( pa.s .) is w'ell ossified ; it is flat below and keeled above, fitting in 
between the trabecular rods. 
The septo-maxillaries, vomers, and labial cartilage (fig. 2, s.mx., v., u.l.) are seen on 
each side and below the septum nasi (s.n.) ; these parts will be fully described in the 
adult stage . 
o 
The single pre-maxillary (px.) is edentulous ; it has ascending, lateral, and palatine 
portions. 
The maxillaries (mx.) are arcuate rods, pointed and bladed behind ; the palatines 
(pa.) are hatchets with a coiled blade and an out-turned, pointed handle ; the ptery- 
goids (pg.) are arcuate and flat, with a spindle-shaped outline ; on their outer edge 
the handle of a small trowel-shaped bone, whose blade is gapped, is attached ; the 
broad end rests on the maxillary. This is the transpalatine (t.pa.). 
The columella (co.) begins to ossify about this time ; this part and the mandible 
will be described in the next or ripe stage. 
Sixth Stage. Ripe Embryos of the Snake, 6^ inches long. 
This is a very important stage ; the osseous centres are all present, and none of 
them ankylosed to others ; the synchondrosial tracts are mere clearly defined divisional 
lines. 
From this stage to the adult few changes of any importance, except in size, occur. 
It is a strong skull even now ; the changes afterwards go to make it one of the 
strongest known ; this is the case in this and other small Ophidia, but in the gigantic 
kinds the skull is a marvel of solidity, there being much ankylosis, whilst the bony 
substance is like ivory. 
The roof-bones have reached the top of the skull, but the fontanelle is still large 
(Plate 31, figs. 4 and 6) ; a side view of the interior (fig. 3) of the skull, whose septum 
nasi has been removed, will display most of the cranial characters. 
Nearly all the ossified chondrc cranium lies behind the eyes ; this is almost entirely 
occipito-otic. 
The basioccipital (figs. 3-5, bo.) is lozenge-shaped and hollow, like the bowl uf a 
spoon. Upon its outer edges there rest three bones — namely, the exoccipitals (eo.), 
