12 THE VOYAGE OF H.M.S. CHALLENGER. 



The thick chondrifying capsule of the ear [cm.) is cut through horizontally in this 

 preparation so as to show the junction of the anterior and posterior canals {a.s.c.,2^.s.c.), 

 and the two ends of Lhe horizontal canal (h.s.c.) 



Between each ear-capsule and the skin there is a large sulcus or ditch ; this runs 

 forwards into a large oval hole, just outside the trigeminal nerve (fig. 5 ; 5) ; the sulcus is 

 the tympanic cavity, and the hole the " Eustachian tube " ; both of these are parts of 

 the first post-oral cleft (PL I. fig. 7, cl. 1). 



The hind basin is short and wide, but the front basin is egg-shaped, with the 

 broad end in front . and the narrow end behind. But this narrow end dips largely, and 

 there it is open below {yy.) ; a little further forwards it is perforated on each side for 

 the large optic nerves (2), and quite at the fore edge the olfactory nerves (1) pass out, 

 and grow downwards and backwards over the nasal space or cleft. 



Between the optic nerves and olfactory sacs the basin has cartilaginous sides, the 

 orbito-sphenoids (o.s.) ; and after a while this cartilage will bridge over the space where 

 the trigeminal nerves run out and connect the orbito-sphenoids to the post-pituitary 

 wall by a temporary alisphenoidal band. 



The middle part is taken up by three cartilages, the trabeculce and intertra- 

 becula ; here the chondrification corresponds very exactly to my first stage in the 

 Selachian (Trans. Zool. Soc, vol. x. pi. xxxv. figs. 3 and 5, tr.). But there the middle 

 part is still soft, and it has not become definite along the middle in the interocular 

 region ; in the Turtle the three bars are coeval. 



The lateral bars are like those of the Tadpole (Batrachia, part 2, Phil. Trans., 

 1876, pi. Iv.) ; but there the middle one develops slowly, and in three distinct parts, 

 ultimately uniting the symmetrical bars. 



In the Tadpole the thin flat intertrabecular floor of the orbital region chondrifies 

 first ; then the meso-ethmoidal wall, and lastly, the anterior space between the cornua 

 trabeculse, where in some kinds ("Hylidse"), it sends forth a well-formed "prenasal 

 rostrum." 



In the Axolotl (" Urodeles," Phil. Trans., 1877, pis. xxii., xxiii.) the trabeculse 

 gradually grow up to the frontal wall, and then become fused together in the inter- 

 nasal space. 



But in the larva of Seironota {Ibid., pi. xxix. figs. 1, 2) they sooner reach the fore 

 part of the head ; in all the " Urodeles " the intertrabecular cartilage is but feebly 

 developed, and that merely as a conjugation of the trabeculse in the nasal region. 



Altogether, whether we compare these things in the Turtle, with types below or 

 above, the intertrabecula has in it a unique development as to relative size and contin- 

 uity, and in its early appearance, contemporaneously with the paired bars (trabeculae). 



All the three rods are nearly circular in section at present (fig. 10), but this is a very 

 temporally state of things, for they all soon grow into vertically compressed plates, and the 



