HOLOCEPHALI. 1 37 



upper Silurian beds to the present period. But none of the 

 palaeozoic fossils are referable to any existing genus. A few 

 only of the mezozoic Plagiostomes, and those chiefly from 

 the chalk, are so determinable : most of them belong or are 

 allied to a family (Cestraciontidce), now nearly extinct. The 

 evidence of the generic forms of Plagiostomes characteristic 

 of the present time become common only in the tertiary 

 periods. No fossil species is the same with any existing 

 one. 



Order II.— HOLOCEPHALI.* 



Ohim ceroid Fishes. 



Char. — Jaws bony, traversed and encased by dental plates; 

 endo-skeleton cartilaginous ; exo-skeleton as placoid gra- 

 nules ; most of the fins with a strong spine for the first 

 ray ; ventrals abdominal ; gills laminated, attached by 

 their margins ; a single external gill-aperture. 



To judge from the paucity of existing representatives of 

 this order of cartilaginous fishes, it would seem, like the 

 Cestracionts, to be verging towards extinction. One genus 

 (Chimcera, Linn.) is founded on a single known species of the 

 northern seas called "king of the herrings" (Chimcera mon- 

 strosa) ; another genus (Callorhynchus of Gronovius) is repre- 

 sented by two known species in the Australian and Chinese 

 seas. The only parts of chimyeroid fishes likely to be fossil- 

 ized are the jaws and spines. The bony and dental substances 

 are so combined in the more or less beak-shaped jaws, that 

 they characterize the order, and are never found separate. 

 It is chiefly on such fossil mandibles, and portions of them, 

 that the evidence of the Holocephali in former geological 

 periods rests. These singular fishes ranged, under different 

 generic and specific modifications, from the bottom of the 

 oolitic series to the present period. 



* Gv. holos, entire ; Jcejrfiale, head : the cranial walls being unbroken. 



