760 MR FRANK J. COLE 
cartilage, and there is always a narrow ring of the same cartilage surrounding the em- 
bedded notochord. Opposite the posterior boundary of the auditory foramen the 
parachordal tube is completed dorsally for a very narrow space, so as to complete the chordal 
roof (cp. fig. 2). This roof is much more extensive in Bdellostoma, so that in this respect 
Mywine is the more primitive. In Myxine, Mitiur* did not find the dorsal fusion of 
the parachordals at all; but I am inclined to think that it practically invariably occurs, in 
spite of the fact that Parxer did not find it also. At the same region independent 
nests of soft cartilage appear within the notochordal sheath, and the notochordal 
membranes almost entirely disappear. The now cartilaginous notochord is, in places, in 
contact with the parachordal tube in which it lies, but there 1s nowhere any fusion 
between them.t The cartilaginous tip of the notochord projects freely beyond the 
anterior border of the parachordals in the median line, as shown in fig. 2; ~The 
parachordals are now supposed to split, and to extend forwards as diverging arms of 
hard cartilage on each side, forming the inner boundary of the auditory foramen (au. f.), 
and meeting the trabeculze opposite the anterior border of the auditory capsule. This 
is certainly the appearance suggested by dissections (fig. 2), but an examination of serial 
sections seems to me to indicate that the so-called parachordal cartilages terminate with 
the soft cartilage—.e. they are composed entirely and only of soft cartilage. For it must 
not be forgotten that the diverging arms are, as far as we can see, nothing more or less 
than the inner wall of the auditory capsule completing the auditory foramen, which is 
presumably a perforation in the wall of the capsule and not an enclosure by the capsule 
with a fused parachordal. Ayers’ and Jackson’s fig. 4 is misleading on this point, as it 
does not show the distribution of the hard and soft cartilage, which seems to me may 
have some significance in this connection. The shading of this region in their fig. 7, 
and also the figure of Rerzius,{ support the view suggested above; but it must not be 
forgotten that the (assumed) complete fusion of the auditory capsule with the trabecula 
in front admits the possibility of a similar fusion of the capsule and parachordal behind, 
although there is absolutely no evidence for it in either case. 
Auditory Capsule (figs. 1 and 2, aw. c.).—This is fused behind and internally with the 
parachordal, as above stated. It is an oval-shaped hollow capsule of hard cartilage, 
sloping upwards and outwards, with its dorsal or inner wall perforated by the large 
ego-shaped auditory foramen (fig. 2, au. f), which is, however, closed by a tough, fibrous 
membrane, about half of which consists of the fibrous cranial wall. This membrane is 
perforated to admit the exit of the auditory nerves. Just opposite the second fenestra 
of the skull (f-*), the dorso-external border of the auditory foramen is connected by 
means of an internal column of hard cartilage with the ventro-external wall of the 
* See his concluding remark, p. 340. 
+ Since writing the above, examination of further series of sections, especially of a vertical longitudinal series, 
indicates that fusion does take place ventrally between the cartilage of the notochord and that of the parachordals. 
In fact, I now question whether the so-called anteriorly projecting tip of the chorda is not after all a part of the 
parachordals. But these questions are difficult to settle with adult material. 
{ Das Gehdrorgan d. Wirbelthiere, i., 1881. 
