DEVELOPMENT OF THE SKULL IN THE URODELES. 181 
pointed behind; and they fill in three fourths of the reentering angle between the 
frontals. 
In front, the premaxillaries (px) do not repeat this, but the nasal process of each 
bone notches the corresponding nasal in its middle. 
There is on each side a third outturned bone parallel with the fore part of the 
nasal and frontal, and articulating with the latter; this is the prefrontal or external 
ethmoidal plate (p.f). Each bone is an arcuato-oblong plate, four times as long as it 
is broad, with irregular edges, and articulating with the frontal within, with the 
parietal behind, and with the back of the ascending plate of the maxillary in front ; 
the outer edge of the bone lies on the nasal roof (na), which appears outside it. 
The likeness to a round arch which is seen in the upper view of the front of the 
skull is increased greatly by the presence of a small, round keystone between the broad 
premaxillaries (px); this is a median ethmoidal ossicle or prenasal bone (p.n.b), the 
true but azygous homologue of the pair of bones wedged in between the premaxillaries 
in Siren (Pl. XXXVIII. fig. 3, e¢.n). 
The premaxillaries have a broad dentary margin one third the extent of that of the 
large maxillaries (fig. 2); above they are scooped for the narial passage (e. 7), they then 
send upwards a broad but sharp-pointed nasal process (7. px), which is let into the nasal ; 
the /efé is larger than the right. Between these thin plates the median nasal passage 
(m.n.p) is seen; in Stren (fig. 5) it lies between the paired dermo-ethmoids. 
The maxillaries (ma) are large falcate bones with a rounded ascending blade 
running up the face; their pointed jugal process is opposite the foremost third of the 
orbital region, and extends some distance behind the broad nasal roof. 
The broad lower edge is covered with a copious rasp of teeth, which extends nearly 
to the end of the bone, and corresponds in front with a similar growth on the palatal 
part of the premaxillaries; the upper surface of these bones is smooth and gently 
convex, the convexity running down from that of the prefrontal. 
The squamosals (sg) are roughly oblong bones with dilated ends. The upper end is 
bifurcate ; but the lower fork, which runs backwards over the side of the auditory 
capsule, is short as compared with what is seen in Menobranchus and Proteus. 
The main lobe above runs obliquely outwards and forwards, and has a ribbed edge ; 
from the hind fork an oblique ridge runs downwards to the lower end, which is 
angular; this lower part strongly clamps the quadrate bone (gq), but it covers very 
little of the upper face of the suspensorium, distally, as compared with the squamosal 
of Siren. 
The parasphenoid (fig. 2, pa.s) is more than equal to both the parietals, covering, as 
it does, the wider surface of the floor as compared with the roof. It reaches beyond the 
basioccipital synchondrosis as a round process; this, by a series of steps, gradually 
enlarges until the bone reaches its widest (basitemporal) part under the foramen for 
the facial nerve (VII). 
2F2 
