38 DEEP SEA FISHES. 
covering the grip in the handles of swords. The scales are very small, they 
are subquadrangular in base and superstructure, and are arranged in regular 
series, in pavement, the individual scales being placed so as to present an 
angle forward and another backward. The outer surface has a low sharp 
keel surrounding a central area; in most cases this keel bears erect irregular 
projections, those at the corners rising highest. Scales that have been 
worn off on the top are shown on Plate IIL., fig. 7; the bases are shown in 
figures 8 and 9. At first sight this type of scale would be described as radi- 
cally different from that of Scymnorhinus, but to a considerable extent the 
differences disappear on closer examination. On the scales of S. dicha the 
superstructure is a sharp pointed and keeled retrorse spine rising obliquely 
from the hinder portion of the base and giving the entire scale a triangular 
appearance. The keel extends from the end of the spine forward across the 
base to the anterior angle. If the keel and the spine are cut away the 
scales are seen to be subquadrangular, somewhat as in Isistius, and their 
arrangement is similar in the two genera. The principal differences are 
those pointed out in the superstructures. The ancestor common to Isistius 
and Scymnorhinus probably had serrated teeth, spiny scales, a smaller 
middle and hind brain, and a branchihyal skeleton more like that of 
Seymnorhinus. 
Lateral System. The lateral system is tubular and is not so complex as 
on many of the other sharks. From the aural region the corporal tube 
bends outward slightly to a point above the base of the pectoral, whence it 
runs directly to the tail, on which, a trifle below the middle of the mus- 
cular portion, it extends about two thirds of the distance from the origin 
of the lower lobe to the notch separating this lobe from the upper. On 
the head, Plate II., fig. 1, the aural tube crosses immediately behind 
the aural apertures. The occipital tube is comparatively long, cranial 
and rostral pass forward with tolerable directness. From the end of the 
occipital the orbital goes down to the angular and suborbital, which 
latter meets the nasal and the subrostral with or without the intervention 
of a short orbitonasal, below the middle of the eye. Immediately behind 
the buccal fold, from the corner of the mouth, the angular joins the oral, 
which continues forward to the end of the lateral lip. Behind the junc- 
tion with the oral there is a short jugular. In front of the mouth the 
nasals meet in a very short median, from which the prenasals diverge 
forward to join the rostrals. The subrostral section of the tube goes 
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