OF THE SKULL IN SHARKS AND SKATES. 197 
But the hindermost of these is not segmented off 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 (7). 
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 vertebra 1. 
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- 
cule and the notochord. They also pass a little beneath the trabecular plates in front; 
for, contrary to my earlier belief, I now find that the trabecule form the ** posterior 
clinoid” region: in the Salmon (“Salmon’s Skull,” pl. 2. fig. 5, and pl. 4. figs. 2 & 3, 
tr, iv), 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, pl. 24. figs. G, H, 1); 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 siz 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. XXXYV. fig. 5, 
and Pl. XXXIX. fig. 6, ne). 
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 
effort to push away the pituitary barrier. 
The interocular plates, or trabeculz, 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. 
1 Professor Huxley’s term “ parachordal” (“On Menobranchus,” p. 198), for these paired bands, is not 
distinctive of them; for the terminal plates of the trabecule are also parachordal. 
VoL. x.—part tv. No. 2.—WMarch Ast, 1878. 2F 
