12 ON THE STRUCTURE OF TRACHYPTERUS ARCTICUS, 
are really inter-branchials, the branchial arches articulating not with them but with 
the cartilaginous and imperfectly ossified elements which alternate with them. 
Gill-rakers are present in two rows on each of the four anterior arches, in one on 
the fifth. They differ in the details of their arrangement from those of Regalecus. 
The first row of gill-rakers stand in a membrane which is attached to the inner 
border of the first arch. In the middle of the series, they are long and irregularly 
grooved ; the lower ones are blunt, and stand free in the membrane without being 
attached to the arch itself (Pl. IL, Fig. 7). They are thirteen in number, and all 
bear on their inner edge small bristly denticles. 
The rakers of the second row alternate with those of the first row, and are 
attached to the posterior face of the arch near to their mner edge. They are 
stronger, shorter, and blunter, and are likewise provided with small sharp teeth, 
which in this case are at the point. They also are thirteen in number. 
On the second arch in the first row, a small raker leads from the second row of 
the first arch and appears on the basal element. All the rakers are attached to the 
arch. They are smaller than the corresponding row of the first arch, but bear small 
teeth, like them, on the inner edge. ‘The first row consists of thirteen rakers, the 
second of ten alternating with the former. 
The third arch has likewise thirteen rakers in the anterior and ten in the 
posterior row. The fourth has nine anteriorly, and posteriorly six only, which latter 
are confined to the cerato-branchial element. The fifth arch possesses eight or nine 
small tooth-like rakers in its anterior and only row. The lower ones are sharp and 
curved. 
The alternation of large and small rakers in each series, which is characteristic 
of Regalecus, is thus not seen in Trachypterus. The immensely long rakers of the 
first arch in Regalecus are not developed, nor do the rakers extend on to the 
pharyngo-branchials of this or of the succeeding arches. 
The Opercular Bones. 
The preoperculum is a simple flat bone, with a three-sided contowr, marked with 
radiating ridges or grooves, thin and papery posteriorly where it overlaps the 
operculum and inter-operculum, comparatively strong im front where it enters into the 
usual relations with the hyomandibular and quadrate. 
The operculum is a much smaller bone, and more vertical in position than in 
Regalecus. It is of an irregular ovoid form, narrower and more solid at the top, 
where it articulates with the hyomandibular, broad and thin below where it overlaps 
the sub-operculum and inter-operculum. 
The sub-operculum is small, and extremely thin:, but appears to be somewhat 
larger and especially broader than in Regalecus. 
The inter-operculum is remarkable for its great length and rectangular shape. 
It ends in front on a level with the preoperculum. 
