DEVELOPMENT OF THE SKULL IN THE URODELES. 175 
Over the foramen magnum they unite to form a rounded process, just as the azygous 
parasphenoid (Pl. XX XVII. fig. 1, pa.s) forms a rounded process under this archway, 
but much further back. On each side, over the broad free end of the hourglass-shaped 
occipital cincture, each parietal sends a bevelled, ovoid, dentate plate; in front of this 
plate each bone is higher and thicker, and at its anterior notched edge articulates 
with the squamosal (sq). Again, in the temporal region each bone is developed into 
a short spur, in front of which, in the postorbital region, the bone bends inwards, and 
then stretches gently outwards to its pointed fore end, where it lies on the ethmoid, and 
binds the outer face of the prefrontal (pf); the sagittal suture is formed by the 
meeting of the raised inner edges of the two bones. 
The frontals (Pl. XXXVI. fig. 1, f) reverse the form of the parietals; they are 
sharp wedges behind and are dilated in front, where they diverge abruptly to let in the 
nasals (n). These bones reach backwards halfway between the foramen opticum and 
the f. ovale (11, v); in front the diverged part binds against the top of the maxillary 
(mx) and the inner edge of the prefrontal (p.f), whilst the nasals (m) overlie the notch 
formed by their divergence. 
The frontal suture has large irregular teeth; and the edges are sharp where the two 
bones are interlocked (fig. 2). 
The nasals (n) together form an oxhead-shaped plate, the cheeks of which articulate 
with the top of the maxillaries, whilst the horns run in between the nasal processes of 
the premaxillaries and the nasal cartilage surrounding the outer nostrils (na, é. 7). 
Their narrow hinder end rests by a squamous suture upon the notched frontals; their 
median suture has strong teeth; their surface is hollow towards the middle, subsca- 
brous, and pitted. 
There are only two pairs of supero-lateral bones, viz. the prefrontals and squamosals. 
The former (p.f) are irregular conchoidal splints, lying outside the fore half of the 
frontals, and articulating also by their front edge with the ascending plate of the 
maxillaries. They are pinched in at the anteorbital region, and only half conceal the 
nasal roof (na); their outer and fore margin is notched; and they overlap the frontals 
by a squamous suture. They are half the size of the frontals. 
The squamosals (sq) are strong, oblong, carinate bones; they articulate above with 
the parietals by a rounded head with raised borders, are crested in a rugged manner 
along their outer surface, and the crest crosses over from the hinder part of the upper 
ribbed edge to the fore edge below. The hind edge is rounded, and the front edge 
rather straight, so that they are semielliptical below; they bind strongly on the half 
bony suspensorium. Here we see the economy used in this group as to bony plates ; 
for this squamosal answers to the proper squamosal (temporal) of most Ganoids and 
of the Siluroid Teleosteans, and also to the preopercular. 
There are only two bones in the marginal arc; for each premaxillary has become 
ankylosed to the maxillary (pa, mx) of the same side; but the former are only one 
252 
