180 THE PLAGIOSTOMIA. 



Centraciontidae. 



The living species of this family are small sharks which have short bodies 

 and heads, blunt snouts, small spiracles below the hinder part of the eye, a 

 narrow mouth near the end of the snout, with about four lobes in each half of 

 the upper lip, both cuspidate teeth and grinders, five gill openings of wliich 

 several are above the pectorals, eyes without nictitating membrane or folds, 

 nostrils connected with the mouth by nasoral grooves, without cirri, two dorsals 

 each preceded by a strong rigid spine, an anal behind the second dorsal, a short 

 deep caudal, small carinate scales, a preorbital articulation between upper jaw 

 and skull, and asterospondylic vertebrae. 



Fossil species were present in the Jurassic or earlier; from what we now 

 know of them they differed comparatively little from the recent forms. 



Name from Kevrpov, a point, spine, spike, or spur, and 'A/ct?. 



Centracion. 



Heterodontus Blainv., 1816, Bull. Soc. philoixi., p. 121 (not Helerodon Latreille, 1802). 



Les Cestracions Cuv., 1817, Reg. anim., 2, p. 129 (not Cestracion Klein, 1742, 1776, or Walbaum, 1792). 



Centracion Gray, 1831, Zool. misc., 1, p. 5. 



Heterodontus, Tropidodus, and Gyropleurodus Gill, 1862, Proc. Acad. nat. sci. Phil., p. 489. 



Head short; snout blunt; crown narrowed, between strong orbital ridges. 

 Eyes small, lateral. Nostrils with two thick valves reaching the mouth and 

 curving toward the grooves. No narial cirri. Mouth narrow, with thick labial 

 folds on both jaws. Teeth alike in upper and lower jaws, cuspidate in the 

 anterior series, elongate longitudinally ridged grinders posteriorly. 



Pectorals large, dorsals moderate, anal small, caudal short. 



The nomenclature is discussed on page 4. 



Base of anal fin two or more times its length from that of the caudal 



origin of first dorsal above the middle of the base of the pectoral, hind margin 

 deeply concave, 

 bands transverse, narrow ..... zebra (page 181) 

 Base of anal little less than twice its length from that of the caudal 



origin of first dorsal above mid pectoral base; fin somewhat concave on 

 hind margin 

 bands both transverse and longitudinal . . philippi (page 182) 



