162 
MR. W. K. PARKER ON THE STRUCTURE AND 
This relative change of the infrahyomandibular has affected the hyoid cornu (%.), which 
has been carried, skullwards, with it. 
Every one is familiar with the manner in which the lower part of the first cleft is 
embraced entirely by the second arch in the Osseous Fishes, the symplectic conti- 
nuation of the hyomandibular bounding this space in front, as the hyoid cornu does 
behind. 
In this stage of the Tadpole there is a partial separation of the symplectic bar, the 
cartilage-cells receding from each other fore-and-aft, leaving a considerable space of clear 
intercellular substance entirely free from cells (Plate VII. fig. 1, sy. ). 
* 
The segment thus partially severed is club-shaped below, and pointed above, where it 
reaches the cupped surface for the hyoid cornu. 
Although there is a rudiment of the symplectic in the third stage (Plate IV. fig. 7, the 
lowest end of h.m.), yet I cannot find that it has developed to any extent, except to 
coalesce completely with the quadrate, even in the fourth stage (Plate V. fig. 1, sy.). 
The free rays which are developed from the first and second poststomal arches to 
form the mandibular and hyoid arches are now much elongated. In the last stage 
Meckel’s cartilage was not half so long as its suspensorium ; they are now equal (Plate 
VII. fig. 1, qu. , mk.). The free rod itself also has undergone a great change of form, 
being thickest now at its articular end ; its fore part was very clubbed in the fourth stage. 
The hyoid cornu (hy.) is not only twice as near the auditory capsule, but it is much 
narrower, and has coalesced with the soft basihyal ( b.h ). 
A deposit of calcareous matter begins now in the roof of the skull in the outer layer 
of the membranous brain-sac ; other tracts appear in the face. These will have a defi- 
nite form in the next stage, and will then be described. 
Sixth Stage. — -Tadpoles with fore legs , and with tail reduced to one-half its former size. 
The last moulting of the epidermis was accompanied by so much absorption of the 
opercular folds as to set the fore limbs free ; and now, the lungs being in function, the 
branchial system is fast diminishing. In this stage I will begin with that which is most 
apparent. We see now a pair of rnembrane-bones on each side of the skull-roof, closing 
in the great “ fontanelle;” these are the frontals and parietals (Plate VII. fig. 3 ,f,p-) ; 
nor are these the only membrane-bones which have been added to the “ parasphenoid 
for in the upper nasal region there is a pair of small semilunar patches, the nasals (n.) ; 
in front the upper labial cartilages have disappeared, and in their place, but not by imme- 
diate ossification of them, the premaxillaries (p.mx.) have arisen; these are short thick 
bars of fibrous bone, having an ascending process, the nasal process, above, whilst on 
the palatal region each bone is knobbed close to its fellow of the other side ; this knob 
becomes the palatal process. 
Behind the premaxillary, and below and outside the palatal bar, is a calcified web, of 
a styloid shape, and having an ascending process behind its first third ; this is the maxil- 
illary ( m.x .). 
