74 MR. W. K. TARKER ON THE STRUCTURE AND 



very much shrunken. The wild and general growth of bony matter, without division 

 into periotic and occipital regions, is now still more remarkable, and, above, the jright 

 and left tracts are rapidly coalescing, so that there is no superoccipital cartilage 

 (Plate 1 1, fig. 1). This bony matter reaches forwards to the great fontanelle (fo.) and 

 laterally up to the inner margin of the horizontal canal (inside t.ti/.). 



Below (fig. 2) there is a wide basioccipital tract {iv.) unossified, and the bony 

 matter only skirts the floor of the vestibule ; it runs, however, well round the pre- and 

 post-auditory nerve-passages (V., IX., X.). 



The steeply-sloping floor of the tympanic cavity (Plate 11, fig. Z,f.ty.) has become 

 much reduced in size, and thus the oval stapes {st.) is seen clearly from below. 



The fontanelle {fo.) has the same relative size, and so have the fronto-parietal bones 

 if-P-) '' ^^^t, the upper part of the skull in the ethmoidal region has altered greatly ; 

 ready to alter still more as the tail, and indeed the whole creature, keeps lessening 

 in size. 



The ethmoidal roof (Plate 11, fig. \,p.e.) has grown over the growing septum nasi, 

 but the nasal roofs are still membranous, or only composed of soft cartilage. 



The back wall of the nasal fossfe is not so distinctly articulated to the edge of the 

 tegmen cranii ; but, gently shelving down, the median part passes into a concavity 

 which lies between the roof and the post-palatine (pt.pa.). The cornua trabeculse 

 (ctr.) have largely united together, but a small hole some distance behind the notch 

 may be still seen : this soon fills in, it i-epresents the large open space in the " Urocleles," 



The post-palatines {pf-pn-) have been, as it were, moulded into a more solid, but 

 altogether a roTmder and smoother, structure ; the sharp crest has become a neat 

 longitudinal ridge, and the hinder process is not a flap but a genicidate projection into 

 the fore part of the suborbital fenestra. 



Laterally (Plate 11, fig. 3, pt.pa.), it appears as a gently concave plate, like a "post- 

 zygapophysis ;" the orbitar process playing against its scooped face. The fore part of 

 the palatine has chondrified the pre-palatine ligament, and the ape.x of this bar abuts 

 against the trabecular cornu {pr.pd., ctr.) ; thus the internal nostril (i.n.) is fairly 

 enclosed by cartilage. 



Beneath (Plate 11, fig. 2), the trabecular, palatine, and suspensorial regions all pass 

 gently into each other ; this, however, is a very temporary condition. 



Besides the general lessening of the cartilage, the suspensorium is not now so bent 

 outwards, and the " pedicle " has bent itself into a sigmoid form for want of room 

 (fig. 2,K). 



The space between the " elbow " of the suspensorium and the spiracular cartilage or 

 otic process {ot.j).) is larger, and that band seems to belong equally to the tegmen 

 tympani and the suspensorium ; the ridged edge of the orbitar process {or.p>.) runs 

 along to the tegmen, strengthening the whole band ; this " extra-suspensorial " tract 

 has no counterpart in the Lamprey. 



The quadrate region {q.) stands out further from the trabecular coiiui, and its 



