548 
ME. W. K. PAEKEE ON THE STEUCTUEE AND DEVELOPMENT 
outer edge of the mandibular suspensorium ; this is the squamosal (sq.), whose name, 
amongst Fishes, is “ prseopercular.” 
On the roof, supplementary to the general deficiency of the cartilaginous “ tegmen,” 
there are four nearly equal, thin shells of subcutaneous bone ; these are the frontals 
and parietals (f, p.) ; in this their early deficiency they still leave a lozenge-shaped 
fontanelle above. 
The next five pairs of bones were present in the last (third) stage ; the foremost of 
these, margining the face in front, are the premaxillaries [px.) ; these bones do not yet 
meet at the mid line, but they have sent upwards a long styloid “ nasal process.” 
The vomers ( v .) have changed but little ; but the palatines are now “ pterygo-palatines ” 
(p-pq-), for the dentigerous plate in the ethmoid region has sent backwards and outwards 
a long, sigmoid, ragged, fenestrate plate, without teeth ; this is the pterygoid process of 
the bone ; it just reaches the quadrate. 
This answers to the “ palato-pterygoid ” bone of the lower Urodeles, as Proteus 
(see Plate 28. p.pg .) and Menobranchus (Huxley, op. cit.) ; and it is also seen in certain 
Fishes whose relationship to the Amphibia is most evident and sure, namely the 
“ Dipnoi.” In the Batrachia, as in the Sturgeon, there is, from the first, a palatine 
bone distinct from the pterygoid ; and in Osseous Fishes a third primary centre, the 
mesopterygoid. 
The dentary and splenial bones on the mandible (d., sp.) have increased in size ; and 
the rod is now ensheathed on its inner side, above, by a rudimentary “ articulare.” 
At this stage we can profitably compare the main nerves with those described by 
Professor Huxley in Menobranchus (Proc. Zool. Soc. 1874, p. 191); one description 
might serve for both. 
The olfactory nerves pass to the nasal sacs beneath the nasal processes of the premax- 
illaries ; they are not yet built into a solid wall of cartilage. 
The optic nerves pass through the sphenoidal crest of the trabeculee, and they serve 
as a landmark to show where the orbital wing begins and where the alisphenoid ends ; 
they are well displayed in the large specimens (Plate 27. figs. 1, 2). 
Leaving out of consideration the small 3rd, 4th, and 6th nerves as of minor importance 
in this research, we find the trigeminal full of interest in this survey. 
As in Menobranchus , “ the orbito-nasal (ophthalmic) division of the trigeminal nerve 
[fig. 5 1 ] passes beneath [the ascending process (a.p.)], which therefore, morphologically 
speaking, ascends higher than the eye, inasmuch as the orbito-nasal nerve, as it passes 
forwards, runs above the optic nerve [Plate 27. figs. 1, 2, 51]. 
“ The orbito-nasal nerve actually leaves the skull by a considerable foramen, common 
to it and the other divisions of the fifth [fig. 5 2 , 5 3 ], which lies between the tra- 
becula internally and below, the prootic [region] externally and behind, and the 
parietal bone above. And this foramen is undivided ; but, as the ascending process of 
the suspensorium passes between the orbito-nasal nerve on its inner and anterior side, 
and the second and third divisions of the fifth on its outer and posterior side, it looks 
