ORDER IL—HOLOCEPHALL (Chimaroid 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 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 (Chinwera, Linn.) is founded on a single known species of the northern seas called " king of the herrings" (Chimaera monstrosa); another genus (Callorhynchus of Gronovius) is represented by two known species in the Australian and Chinese seas. The only parts of chimwroid 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 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.
Genus CHIMERA. —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 laminar which do not extend to the margin. The maxillary dental plates, one in each bone, are triangular, and present a broad surface to the lower jaw.
Genus Iscmonus, Egerton.—Each upper maxillary has four dental columns ; the lower jaw is less produced and deeper than in Edaph,odus. Of this genus, I. Johnsoni is from the lies of Dorsetshire ; I. Egertoni from the Kimmeridge of
Shotover ; and I. Townshendi, a magnificent species, from the Portland stone. Two species (I. Agassizii and I. brevirostris) are from the cretaceous beds ; at which period the genus appears to have perished.
Genus GANODIIS, Egerton (including Ganodus and Psit tacodus of Agassiz).—This genus is exclusively represented by species from the oolitic slate of Stonesfield—e. g., G. Buck landi, G. Colei, G. Owenii.
Genus EDAPHODUS, Egerton (including Edaphodon and Passalodon of Buckland).—Each upper maxillary has three dental columns ; the lower jaw is more produced, but less deep, than in Ischiodus : the premaxillary dental mass consists of five vertical and slightly bent series of oblique and curved transverse plates ; the median and longest series being strength ened by a supplementary dental column behind : it represents the genus Passalodon of Buckland. The large E. Sedgwickii is from the greensand near Cambridge ; the still larger E. gigas from the chalk of Kent and Sussex. The ichthyodorulite called Psittacodus Mantelli by Agassiz may be the dorsal spine of this species. Three species, including the E. Bucklandi, are found in the eocene of Bagshot and Bracklesham ; and one species (E. helvetieus) is from the mollasse of Switzerland.
Genus ELASMODUS, Egerton.—Each upper maxillary has three dental columns, but the dentine is confluent, " being rolled round like a scroll on the substance of the bone, one edge forming the margin of the tooth, the other buried deep in its The premaxillary has a thin incurved scal priform tooth, rounded at the cutting edge, of a lamellate struc ture, with a columnar arrangement of the plates, which are juxtaposed. This genus is exclusively represented by species —e. g., E. Hunteri—from the London clay of Sheppy.