OF THE MARSIPOBRANCH FISHES. 
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first, and is larger, often much larger, than the para-chordal ; moreover, in most 
low forms the prochordal part is developed first. The notochord is very uniform 
in bulk along the spinal region in these large long Fishes, only gradually lessening 
in the caudal region. In the head, this rod (Plates 16, 17, nc.) suddenly becomes 
conical, and ends between the middle of the auditory capsules (cm.) so that this part 
of it is only one-tenth the length of the entire skull. Of course in the embryo 
the relative length was much greater. In the embryo of the Lamprey, as I shall 
show in my Second Part, it is half as long as the entire skull, and in the youngest 
chondrocranium figured by me in Lepidosteus (Phil. Trans., 1882, Plate 30, fig. 3) it is 
three-Jifths the length of the skull. 
So that we see that in these low Myxinoids—the lowest known Vertebrata—the 
chondrocranium, during development, must have undergone a large amount of change, 
and we seem to be almost as far off as ever, in these types, in their adult state, from 
finding an archaic skull. Embryonic chondrocrania, in various Ichthyopsida, are 
our best guides. The investing mass (Plate 17, figs. 2, 3, iv.) is complete for a short 
distance, both above and below, and it projects, as hard cartilage, a little distance 
behind the auditory capsules, thus forming a rudimentary basi-occipital; but there is no 
occipital arch or ring. This short parachordal tract is round behind, and deeply notched 
in front, for the trabeculae begin opposite the middle of the small auditory capsules. 
Those bands are only free from surrounding parts for a very short distance, being 
first (at then parachordal roots) continuous with the infero-internal edge of the audi¬ 
tory capsules, then on the outside with the pedicle, and in front of the short oval 
subocular fenestra ( s.o.j '.), for the remaining two-thirds of their length, they are 
continuous with the palatines (pa.tr.). This latter essentially compound part is very 
solid, and for the last sixth of the skull-length the two sides are coinpletely united, so 
as to form a very solid ethmoidal region (eth.). The whole roof is unfinished, being only 
closed by the dura mater, but the depth of the auditory capsules, and the thickness of 
the palato-trabecular bars, give a trough-like character to this flat, unfinished skull. 
Below (Plate 17, fig. 3), the main bulging is formed by the hinder intertrabecula 
(p.i.tr.) and the auditory capsules, which project almost as much below as above ; 
the skull proper is gently convex in the fore part, and in front bends downwards 
a little. The so-called “pituitary space,” or lower cranial fontanelle, reaches from the 
middle of the auditory capsules to the ethmoid, and is lanceolate, but its narrow fore end 
is rounded. About half this membranous space is floored by the soft convex hinder 
intertrabecula (p.i.tr.), an additional cartilage, two-thirds the length of the skull, and 
which reaches from the trabecular roots to the ethmoidal commissure. It is a ladle, 
with its handle dilated in front, and its bowl produced behind, into a short second 
process or beak ; the bowl occupies about a fourth of its length, and is near the hind 
part. The whole upper part is hollow (Plate 17, fig. 2, p.i.tr.), and at the middle of 
the bowl a short side band runs into the trabecula right and left, these bars being 
soft for a short space, where the junction takes place, and at this part the trabecula? 
