2 ON THE STRUCTURE OF TRACHYPTERUS ARCTICUS, 
can, however, trace in it all, or almost all, the elements usually present m a 
Teleostean skull. 
The parts originally preformed in cartilage still retain in the adult a large 
amount of this cartilage unossified; the membrane bones are for the most part 
greatly expanded, are characteristically sculptured, and those of them which lhe 
superficially are covered by so thin a layer of silvery skin as to be easily 
recognizable in the undissected fish. 
The cranium or brain-case is relatively very small in proportion to the general 
bulk of the fish, and to the great size of the jaws, and of the hyoid and opercular 
regions. Posteriorly the cranium is elevated in a high supra-occipital crest, which 
in Regalecus is flat and smoothed away; while below the foramen magnum a 
descending crest or keel, not quite so conspicuous as in Regalecus, is formed 
principally by the basioccipital and prootic bones. The orbit is immensely large, 
and the inter-orbital region very incomplete: the alisphenoids and orbitosphenoids 
meet together below the floor of the anterior part of the brain-case, in which the 
basisphenoid takes no part, while the presphenoid is altogether absent. The 
ethmoidal region is bent. abruptly downwards, and the parasphenoid coming into 
relation with it anteriorly and with the basioccipital keel behind, forms a bridge 
even more than usually distant from the true base of the cranial cavity. 
The roof of the skull, in front of the supra-occipital, constitutes a deep groove, m 
which long processes from the premaxillee slide in the act of protrusion and retrac- 
tion. This groove is mainly formed by a cartilaginous roof or tegmen crani, in which 
is developed the large mesethmoid. The frontals are thus separated from one 
another to form great part of the walls of the groove, above which they rise in 
high ridges, nearly on a level with the supra-occipital posteriorly, but declining to a 
point in front. 
The region of the auditory capsule is prominent, but is destitute of marked 
ridges or angles. Thus even the pterotic, usually salient, here forms only a small 
rough patch on a level with the surrounding bones. 
The long processes of the premaxille, further elongated by a continuation 
backwards in cartilage, are attached to a loose fold of integument, which posteriorly 
is attached chiefly to the pre-orbital bones. In this fold the processes of the pre- 
maxillze (and of the maxillz which are attached to them) can be thrust backwards 
till the end of their cartilage continuation reaches the supra-occipital: in the 
protracted state it is on a level with the anterior end of the frontal. This whole 
mechanism is automatically pulled forwards as the mandible revolves upon its 
articulation with the quadrate, and is further depressed and withdrawn by move- 
ments of rotation at the palato-ethmoid and hyomandibular articulations. Its 
mouth, which, when closed, was situated at the extreme anterior and upper angle of 
the head, and was scarcely noticeable there, becomes when open a long, narrow 
vertical slit. 
