37 
three pieces, which represent the pharyngo-branchials of 
the three arches to which they belong. These pieces are, 
however (and especially the anterior two), so closely bound 
together that they may, as we have known them to do in 
other forms, fuse up in old fish. The second pharyngo- 
branchial (P.Br.?) is a stout laterally compressed bone 
articulating with the third pharyngo-branchial posteriorly. 
It bore five teeth in one row. 
Basi-branchial III. (2.Br.°). A very thin laterally 
compressed bone, apparently wedged out of existence by 
the large hypo-branchials II. Its posterior extremity lies 
under and is covered by the two hypo-branchials III. 
Branchial Arch III.—The hypo-branchial is smaller 
than in arch II., but the ventral spine is both larger and 
longer, and articulating strongly with the same spine of 
the other side forms a bony arch traversed by the ventral 
aorta. The anterior spine in hypo-branchial II. is absent. 
The cerato-branchial is grooved ventrally as in arch IL., 
but more deeply. The epi-branchial bears two large 
tuberosities at its distal extremity. The posterior of these 
articulates with the pharyngo-branchial III. (P.Br.*), the 
anterior by two strong ligaments with the epi-branchial 
IV. The pharyngo-branchial (?.6r.%) bears a strong 
process behind for articulation with the pharyngo- 
branchial II., and bears eight teeth in two rows. 
Basi-branchial IV. (B.Br.')—A very small nodule 
of cartilage wedged in between the bases of arches III. 
and IV. It is only connected with the fourth arch on the 
ocular side, the basal elements of this arch on either side 
meeting in the mid-ventral line. The morphological 
value of this cartilage cannot be determined on adult 
material. It is obvious that the branchial arches have 
undergone reduction from behind forwards. ‘Thus there 
are only three segments in the fourth arch. Now it is 
