PISCES. 327 



ORDER VIII. PLAGIOSTOMI. 



Branchiae pectinated, fixed, opening outwards by seve- 

 ral distinct apertures : no opercle : jaws represented 

 by the palatine and postmandibular bones, which 

 alone are armed with teeth : pectorals and ventrals 

 always present; the latter (in the male) furnished 

 on their internal margin with long appendages. 



73. SQUALUS Body elongated : tail thick and mus- 

 cular : pectorals of moderate size : branchial openings at the 

 sides of the neck: snout more or less produced; with the 

 mouth and nostrils beneath : eyes lateral. 



* Snout short and obtuse: nostrils near the mouth, with 

 a groove-like prolongation conducting to the edge of 

 the lips ; more or less closed by two membranous Jlaps ; 

 caudal truncated at the extremity, not forked. 



(1. SCYLLIUM.) Teeth sharp and pointed, with small denticulations 

 on each side : temporal orifices, as well as an anal fin, always 

 present : dorsals very much behind, the first never in advance of 

 the ventrals : branchial openings partly above the pectorals. 



** Snout produced : nostrils not prolonged in a groove, 

 nor furnished with valves : caudal with a lobe beneath, 

 causing it to appear forked. 



(2. CARCHARIAS.) Snout depressed; the nostrils beneath the middle: 

 teeth cutting, pointed, and generally denticulated at the sides : 

 first dorsal far in advance of the ventrals; second nearly opposite 

 to the anal, which is always present : no temporal orifices : the 

 last of the branchial openings above the pectorals. 



(3. LAMNA.) Snout pyramidal ; the nostrils beneath the base : teeth 

 sharp and pointed, denticulated at the sides : first dorsal consi- 

 derably before the ventrals: anal present : no temporal orifices : 

 the branchial openings all before the pectorals. 



(4. GALEUS.) Snout depressed; the nostrils beneath the middle: 

 teeth sharp and pointed, with a few denticulations on the outer 

 side only : temporal orifices, as well as an anal fin, present : the 

 last branchial opening above the pectoral. 



(5. MTJSTELUS.) Snout depressed ; the nostrils beneath the middle ; 

 teeth small and obtuse, forming a closely-compacted pavement 

 in each jaw : temporal orifices, and anal, both present. 



