DEVELOPMENT OF TUE SKULL IN THE UEODELES. 181 



pointed behind; and they fill in three fourths of the reentering angle between the 

 frontals. 



In front, the premaxillaries (jkv) 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 {jJ-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 (^J.r) ; 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, etji). 



The premaxillaries have a broad dentaiy margin one third the extent of that of the 

 large maxillaries (fig. 2) ; above they are scooped for the narial passage {e. n), they then 

 send upwards a broad but sharp-pointed nasal process (». jJi'), which is let into the nasal ; 

 the left is larger than the right. Between these thin plates the median nasal passage 

 (m. n.p) is seen; in Siren (fig. 3) it lies between the paired dermo-ethmoids. 



The maxillaries (mx) 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 (sq) 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 {q), but it covers ^'ery 

 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 



