276 PEOF. W. K. PAEKEE ON THE STEIJCTUBJB AND 



The shaft of the columella (medio-stapedial, m.st) turns inwards and a little backwards, 

 and the dilated stapedial end fits into the oblique fenestra ovalis (see PI. LXV. 

 figs. 3 & 8). 



Over the temporary and imperfect joint of the columella, but arising most from the 

 upper part, we see a triangular ascending process (s.st) on which the large reniform 

 suprastapedial (s.st) rests, and from the lower side, in front of the segmentation, the 

 extrastapedial gives ofi" a small curved spur, which is hooked backwards ; this is the 

 infrastapedial process [i.st). Attached to the descending process of the extrastapedial 

 (infrastapedial) we see a small curved segment of cartilage ; this is the " epihyal " {e.hy), 

 a distinct piece, a distinct centre in the Ganoid and Teleostean Fishes. 



This small epihyal conjugates the distal pharyngo-hyal piece, or extrastapedial, 

 with the main hyoid bar, the ceratohyal {c.hij) ; it is attached to its " shoulder." The 

 main bar then descends as a considerable cartilage, but less than a fourth the bulk of 

 the quadrate, and is fused with the mandible behind its articulation with the quadrate ; 

 this bar follows the curve of the quadrate. 



We can describe the distal part of the hyoid arch in the next stage (see PI. LXV. 

 fig. 4), where it will be seen to be greatly developed as compared with what is seen in 

 the second stage (PI. LXVIII. fig. 7). 



I shall take up the stapedio-hyoid chain again after describing the other parts of the 

 skull in their various changes. But it will be necessary to reexamine the metamorphosis 

 of this curiously modified branchial arch when I come to summarize the whole. 



c. Investing Bones. 



Before describing the investing bones I will call attention to the structure of the 

 palate, as displayed in the lower view of the head of an embryo 2 ^ inches long, the 

 lower arches of which had been removed. In this state (PI. LXIV. fig. 7) the mem- 

 branous palate has become very extensive, but the right and left selvedges have not 

 formed a perfect seam. The hinder third is open, and in the front part of this open 

 space the middle (or internal) nasal opening {i.n) can be seen, right and left of the fold 

 of skin that covers the base of the orbital septum. 



The hinder third of the notch is notched again ; here the posterior narial opening 

 passes into a rounded narrow median space, in which the right and left tympanic clefts 

 (c/') meet. These sigmoid passages can be seen to terminate externally behind the 

 quadrate cartilage [q). 



Here we have the rudimentary condition of the double tympano-Eustachian labyrinth 

 of the adult Crocodile. The Eustachian openings have melted into one at the mid 

 line as in the "Batrachia Aglossa" (see Giinther's "Batrachia Salientia," 1859, p. 1, 

 and my "Batrachian Skull," part 2, Phil. Trans. 1876, pi. 59. fig. 2). In the earlier 

 stages (PI. LXIII.) the Eustachian openings [cl^) are far apart, as in the Batrachia, 

 generally. 



