I 1 (J PALEONTOLOGY 



Order II.-HOLOCEPHALI. 

 ( CJi im ceroid Fishes . ) 



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

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

 ules ; most of the fins with a strong spine for the first ray ; 

 ventrals abdominal ; gills laminated, attached bj^ 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 toAvards extinction. One genus (Chimcera, Linn.) 

 is founded on a single known species of the northern seas 

 called " king of the herrings" (Chimcera monstrosa) ; another 

 genus (Callorhynchus of Gronovius) is represented by two 

 known species in the Australian and Chinese seas. The only 

 parts of chimseroid fishes likely to be fossilized 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 

 Holoccphali 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. 



Genus Chimcera. — The premaxillary teeth, one in each 

 bone, are oblong, about twice as high as they are broad, and 

 terminate below in a transverse trenchant edge ; they present, 

 exteriorly, vertical columns of alternately harder and softer 

 substance, occasioning a notched margin when worn by use ; 

 interiorly, they have oblique laminae which do not extend to 

 the margin. The maxillary dental plates, <mr in each bone, 

 are triangular, and present a broad surface to the lower jaw. 



Genus Ischiodus, Egerton. — Each upper maxillary has 

 four dental columns ; the lower jaw is less produced and 



