KO) ON THE STRUCTURE OF TRACHYPTERUS ARCTICUS, 
backwards a process like that of the pre-maxilla, but much shorter. The maxilla 
bears no teeth. 
The Hyoid Arch (Pl. I., Fig. 2). 
The constituents of the hyoid form a small but stout arch. Of the six 
branchiostegal rays, two are attached by cartilage to the epi-hyal, and four to the 
cerato-hyal. The rays are long, curved, and hollow, ending in the edge of the 
branchiostegal membrane. 
The inter-hyal is a very short rod, possessing a nodule of cartilage at each end, 
and connects, in the usual way, the cartilage between the hyomandibular and 
symplectic with the epi-hyal. 
The epi-hyal is a rather large, flat, and thickish bone, with a rounded and 
prominent posterior border. 
The cerato-hyal is large and flat, constricted in the middle, much expanded at 
the top, slightly so below. The anterior and upper angle is very prominent, over- 
hanging the larger of the two hypo-hyals. The cartilage between this and the 
preceding element diverges posteriorly, and supports four of the branchiostegal rays. 
The lower edge is rounded, and is capped by cartilage which contains the two 
hypo-hyals. 
Of the two hypo-hyals, the upper one is much the larger: it is attached in its 
whole length to the anterior border of the cerato-hyal. The smaller hypo-hyal is a 
little crescent of bone, with a free inferior edge, from which a short ligament proceeds 
to join its fellow from the opposite side, and thus give attachment to the uro-hyal. 
The wro-hyal is a large vertically flattened, median bone, with a somewhat 
pointed anterior end, from which strong ridges radiate backwards. 
The Branchial Arches (Plate I., Figs. 7 and 8). 
There are five pairs of branchial arches, which, in the general arrangement, 
the mode of their segmentation, and the disposition of their ventral median series of 
copule, agree for the most part, but not altogether, with those of Regalecus. All 
bear gills, with the usual exception of the fifth, and the gill-slits are all surrounded 
by a row of gill-rakers. 
The first four arches contain each the usual four elements, viz., hypo-branchial, 
cerato-branchial, epi-branchial, and pharyngo-branclual; the fifth is, as usual, 
unsegmented, constituting the pair of so-called hypo-pharyngeal bones. 
The first arch is the longest. Its hypo-branchial is a long three-sided rod, 
attached below to the cartilaginous third median element. The cerato-branchial is 
broad, and deeply grooved posteriorly for the branchial vessels. The epi-branchial, 
also grooved posteriorly, is a flattened bone, narrow at its articulation with the 
cerato-branchial, but broad and rounded at its proximal end; here is attached to it 
the long needle-shaped and rudimentary pharyngo-branchial, which lies over and in 
