18 ON THE STRUCTURE OF TRACHYPTERUS ARCTICUS, 
The post-temporal is firmly attached to the back of the skull. It lies in a 
recess covering the parietal, and bounded internally by the epiotic and exoccipital, 
externally by the frontal and pterotic (Pl. 1, Fig. 4). It is an elongated, flattened 
bone, slightly hollow on the anterior face, which is applied closely to the skull, and 
convex on the posterior or external face. It is pointed anteriorly, and has longi- 
tudinal grooves and ridges running down its length. Below it is thickened, and 
presents an articular surface at its base for the head of the supra-clavicle. 
The supra-clavicle is a long straight bone, articulating above by a rounded, 
thickened head with the post-temporal, and applied below by a long splint-like 
surface to the outer face of the clavicle. The bone is broadest in the middle and 
pointed below; it is longitudinally grooved on the outer surface, and is somewhat 
hollowed within where it fits on to the clavicle. 
The clavicle is a long, slender, curved bone. The upper end is pointed, and 
continued downwards from it are two grooves on the posterior side, separated by a 
sharp ridge or keel; the outer of these grooves receives the supra-clavicle, the inner 
one is longer, and extends to the angle of the bone. The lower extremity is also 
pointed, and meets its fellow in the middle line. 
The post-clavicle is a long slender bone, which extends backwards and down- 
wards from the clavicle, and is firmly attached by ligament to the inner of the two 
grooves on the posterior surface of that bone. The post-clavicle is about two and 
one-fourth inches long, much shorter, that is to say, than in Regalecus. 
The coracoid is a flat triangular bone, slightly twisted, with an uneven sculptured 
surface. Its base is partly hidden by the cartilage uniting it with the scapula and 
brachials. It is not united, as in Regalecus, by its anterior border to the clavicle, 
but ends below in an elongated spine, which is attached by ligament to the extremity 
of the former bone. 
The scapula lies in the cartilage already mentioned. It is a very small, flattened 
nodule, with rounded corners, and bearing a foramen a little anterior to its centre. 
The brachials are three very small, thin, flattened bones. Between the first or 
anterior and the second two small foramina exist in the cartilage: the second and 
third are in close contact. The first is much the smallest. They seem to be 
rather smaller than in Regalecus, and contrast more with the scapula than in that 
genus. 
The pectoral fin consists of twelve unjointed rays. Of these, the first three 
articulate with the scapula, and only the last with the third or posterior brachial. 
The pelvic fin.—Between the coracoid and post-clavicle on each side we found 
a number of elongated, hollow ossifications. These were about one-sixteenth of an 
inch in diameter, but broken in both our specimens. A comparison with Regalecus 
leads us to the belief that these are all that represent the pelvic girdle and fin. 
According to LurKEN, it is always rudimentary or absent. 
