EEPOET ON THE GREEN TURTLE. 45 



the back part of the huynx, the basi-hyal, and the cerato-hyals {lx.,b.hy.,c.hy.) are also 

 cut across. 



Eleventli Section. — We now reach the auditory region (fig. 4), and from the obliquity 

 of the head the cranial roof cartilage {s.o.) is seen, the alisphenoidal selvedge {al.s.) 

 running from it into the auditory capsule {au.). The anterior canal (a.s.c.) is severed 

 near its ampulla, and a little of the general cavity of the vestibule {vh.) is also shown. 

 The capsule runs into the basal plate below, the isthmus of cartilage under these 

 parts passes inside the trigeminal nerve (5), the internal carotid {i.e.) is also seen in 

 its canal, and both these structures lie on the pterygoid (jjg.). At the middle of the 

 floor the bony matter of the basisphenoid (h.s.) is seen enclosing the cartilage from 

 which the apex of the notochord has shrunk. 



The quadrate {q.), half bony, is cut through in front of its tympanic cavity exactly at 

 the hinge, so that here the outer " articular " bone is seen to invest the cartilage of the jaw 

 within very closely. Here, exactly in front of the tympanic cavity, the c[uadrato-jugal 

 {q.j.) is severed ; between it and the post-orbital {pt.o.) there is a mere membranous 

 space. Here the parietal {p.) is bifoliate, part going over the temporal muscle {t.m.) to 

 the post-orbital, and part going dowai to protect the sphenotic tract of cartilage. 



Below, behind the tongue, the trachea, cerato-hyals, and hypo-branchials {trc.,e.hy., 

 h.hr.) are cut through. 



Tioelfth Section. — A little further backwards we come across new parts (fig. 5) ;. the 

 brain is shown as retained in the cranial cavity, and the optic lobes (C 2) are seen 

 lying over the fore part of the medulla oblongata (C 3) ; but the cerebellum does not 

 come into view. 



The thick cartilaginous roof is now superoccipital (s.o.), and this runs into the 

 widening auditory capsule, in which we see the arch of the anterior, and the ampulla 

 of the horizontal canal {a.s.c, h.s.c.) ; the eighth nerve (8) is seen passing through the 

 meatus internus, and the columella {co.) filling the fenestra ovalis {co.,f.o.), which 

 leads to the large vestibule {vb.). This section is directly in front of the small 

 budding cochlea. 



The seventh nerve (7) is seen riding over the columella {co.), beneath which 

 the pterygoid bone {pg.) creeps outside the canal formed by it for the internal carotid 

 artery {i.e.) ; foi' this bone has a most extraordinary development in the Chelonia, just 

 as the " basi-temporal " plates have in the Bird. These are not homologous parts, but 

 they are vicarious of each other in function in these two types. 



Here, in the fore edge of the basioccipital bone, the essentially homologous nature 

 of the inter-segmental vertebrae, and the non-segmented basis cranii in its hind part, is 

 well seen in this section. 



The notochord (figs. 5, 5") is here pinched, laterally, in a conical mass of true 

 cartilage,— the mesoblastic sheath of the chord, — and embracing these on each side, we 



