ON THE GENERAL MORPHOLOGY OF THE MYXINOID FISHES. 773 
it is seen that the narrow trench in which these parts play is formed by the posterior 
continuation of the paired rails described above, which are here much deeper vertically 
and are raised up more perpendicularly from the basal plate. In this way a deep 
narrow trough arises—the sides represented by the paired rails and the floor by the basal 
plate. Posteriorly, as above mentioned, this trough transmits the tendon of the M. 
longitudinalis lingue, which is thus kept in the middle line. Here the rails become 
almost entirely fibrous, and finally pass over into the fibrous roof of the canal formed by 
the posterior segment of the basal plate, as described and figured for Bdellostoma by 
J. MtuiEr. The rails therefore represent the bifurcated fibrous roof of the posterior 
seoment of the basal plate continued forwards over the dorsal surface of the middle 
and anterior segments, as is evident from their relations and histology. The appearance 
of soft pseudo-cartilage in them thus corresponds to the existence of the same tissue in 
the tendons of some of the muscles. The postero-external angles of the middle segment 
receive the ventral extremity of the first branchial arch, consisting of soft cartilage, which, 
however, gradually mixes with the hard cartilage of the basal plate without a break. 
When the detached lower division of the second branchial arch is present (fig. 1), it fuses 
with the first arch shortly before the latter reaches the basal plate, as described for 
Bdellostoma by AYERS and Jackson. 
The posterior segment (b. p.’) is about half again as long as the other two segments 
together, and is immovably attached to the middle segment. Even here, at the 
junction-place of two skeletal tissues of different character, it is not possible to establish 
a joint. The posterior border of the middle segment is ragged, and bears small out- 
growths of soft cartilage. The matrix of the posterior segment is very closely connected 
with this border, also contains nests of soft cartilage, and even in parts seems to be in 
direct organic connection with the middle segment. The posterior segment is formed 
of a thick sheet of hard pseudo-cartilage bent up longitudinally at the edges so as to 
form in transverse section the figure of a U, and roofed over dorsally in front by fibrous 
tissue, as above described, and behind by the anterior extremity of the M. copulo-copularis. 
Its cavity transmits the tendon of the M. longitudinalis lmgue. Posteriorly, the segment 
narrows down in the vertical plane; the sides of the U first of all diverge and then 
disappear, and in this way leave only the solid keel, which tapers down to a point and 
vanishes. 
In Bdellostoma, according to J. Mt.umr, the anterior segment of the basal plate 
consists of fowr pieces, the internal bar being divided in the middle line, and the two 
halves connected by ligament. Further, it is separated by a movable joint from the 
middle segment, the latter segment in its turn consisting of two pieces meeting at a 
median suture. This description is confirmed by AyrERs and Jackson, except that 
the inner bars of the anterior segment are stated to be fused in the middle line 
anteriorly, where, further, they are divided by a transverse suture. Again, according to 
Ayers’ and JacKson’s figure and apparently their description, the lateral labial cartilages 
are not fused to the external bars of the anterior segment but are only “attached ” 
TRANS. ROY. SOC. EDIN., VOL. XLI. PART III. (NO. 30). 114 
