IRISTIUS. 33 



the tail. Its body is r.atlier long and rounded, its head is small and taper- 

 ing ; it has moderately large spiracles and narrow gill openings ; and its 

 teeth differ greatly in shape and mobility on the two jaws. Its fins, except- 

 ing the caudal, are all small ; the dorsals differ little in size or shape and 

 both are behind the middle of the body. The eyes are large, .the nostrils 

 are small and the body is phosphorescent. The single known genus is 

 probably nocturnal and descends to great depths, though not an inhabitant 

 of the bottom. 



IsiSTius. 



iSct/mnu.':, Quoy and GaimarJ, 1824, part. 



Leius Kner, 1865, Denkschr. Ak. Wieii, XXIV., Extr. p. 9 (Nov. 10, 18G4), cliaracterized. 



Isistius Gill, 1805, Pr. Pliil. Ac, 2G4 (Nov. 22, 1804), named. 



Body elongate, subcylindrical, tapering backward, abdominal cavity long. 

 Head moderate in size, narrowing forward, depressed. Snout subcouical, 

 blunt. Nostrils anterior. Mouth inferior, transverse, with a deep groove 

 in front of the upper jaw and a deep fold behind each angle. Lips well 

 developed ; lower labial fold at the angle of the mouth, short, rounded, 

 valvular. Teeth in the upper jaw small, raptorial, erectile, lanceolate, those 

 of several series in function at once. Teeth of the lower jaw large, sectorial, 

 erect, fixed, blade-like, the single series in function forming a continuous 

 serrate-edged plate. Eye large, lateral ; orbit circular in front, angled 

 behind ; no nictitating membrane. Spiracles medium, on the top of the 

 neck. Five small gill openings, not in a groove. No anal fin. Dorsals 

 small, without a spine, similar, anteiior backward of the middle of the body. 

 Caudal short and deep. A short dermal fold on the side of the tail. Lat- 

 eral system tubular. Stomach very long ; intestine short, with a spiral 

 valve. Scales small, depressed, in pavement. 



So far as yet determined the range of the known species extends through- 

 out tropical seas to fifty-five degrees from the equator. 



The generic name is given as accepted by others, though the records 

 appear to favor the name Leius given by Kner. 



