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 III., 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. Ucha 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 ai'e 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 

 Scymnorhinus. 



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. Prom 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 tlie 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 thei'e 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 



