or THE SKULL IN SHARKS AND SKATES. 197 



But the hindermost of these is not segmented oif at present from the rest of the axis ; 

 this part is composed throughout of two tapes of young cartilage, closely applied to the 

 sides of a median rod — the notochord (nc) — whose diameter is one third of that of 

 either lateral band (^ v). 



I have traced these structures back behind the inter-auditory region more than twice 

 as far as that region extends, without finding any transverse segmentation answering to 

 vertebral division : hence we are perfectly safe in assuming that the "basilar plate," 

 or investing mass, is a continuation of the substance which in the spine makes itself 

 into vertebrae ^. 



The inter-auditory part of the investing mass has its sides bevelled and crescentically 

 notched or concave ; and the outer edges pass to some extent beneath the capsules. 

 They do not reach further forwards than to the first third of the capsules, but are 

 larger in the middle than at the sides, being inwedged between the ends of the trabe- 

 culse and the notochord. They also pass a Uttle beneath the trabecular plates in front ; 

 for, contrary to my earlier belief, I now find that the trabeculse form the " posterior 

 clinoid " region : in the Salmon (" Salmon's Skull," pi. 2. fig. 5, and pi. 4. figs. 2 & 3, 

 tr, i v), the ends of the delicate trabecular band lie over the fore ends of the basilar 

 plates. 



The cephalic part of the notochord has not yet lost the bend downwards which is so 

 conspicuous in early stages (Balfour, No. 2, pi. 24. figs. G, H, I); but it is much 

 straighter than when first distinguishable. 



At present, instead of ending in a hooked down-turned point, it ends in a beaded 

 manner against the back of the pituitary body (py), which gets somewhat under the 

 notochord which grows obliquely downwards and backwards (PI. 'XXXIV. fig. 5, py). 



The end of the notochord, where it pushes against the pituitary body, is vesicular ; and 

 behind this terminal swelling there are six more similar moniliform enlargements ; the 

 second and third are small, and lie in a twisted manner in front of the remaining four, 

 which become as large in diameter as the even part of the rod (PL XXXV. fig. 5, 

 and PI. XXXIX. fig. 6, nc). 



This beaded condition of the fore end of the notochord appears to me to be open to 

 two interpretations : it may be a temporary subsegmented condition, corresponding to 

 undeveloped or suppressed segments in the head ; or it may simply be a puckering or 

 folding of the sheath in a vegetative attempt to grow further forwards, the result of an 

 efi'ort to push away the pituitary barrier. 



The interocular plates, or trabeculse, are the parts hardest to be understood. They 

 may be precociously solidified tracts of the same nature as the investing mass, having 

 the notochord between them only behind, and their separateness due to a dislocation, as 

 it were, the result of the mesocephalic flexure. 



' Professor Huxley's term "parachordal" ("On Menohranclms," p. 198), for these paired bands, is not 

 distinctive of them ; for the terminal plates of the trabeculse are also parachordal. 



VOL. X. — PAKT IV. No. 2. — March 1st, 1878. 2f 



