950 CLASS PISCES. 



massive teeth, which are strictly comparable to those of the Dipnoi, 

 constitute the whole of the solid part of the jaws. 



This order may be regarded as in some respects connecting the 

 Elasmobranchs with the Dipnoi — the autostylic cranium and the 

 dentition being essentially Dipnoid. Chimseroids have existed from 

 the Lias upwards, and not improbably date from the Devonian ; 

 while, as is usually the case, some of the extinct genera show much 

 more generalised affinities than their existing representatives. 



Family Squaloraiid^e. — The extinct genus Squaloraia was long 

 regarded as an Elasmobranch of somewhat uncertain affinities ; but 

 according to the observations of Dr Traquair, it should find a place 

 in the present order. In this genus, which is confined to the Lias, 

 there is an elongated body; while the skull is produced into' a long 

 flat rostrum, and carries a basal pair of teeth separated at the sym- 

 physis, in advance of which are two small vomerine teeth of the 

 normal Chimaeroid type. Further, the skull of the male has a pre- 

 hensile spine on the upper part of the snout, resembling in structure 

 that found in Ischyodus. The " lateral line " agrees with that of 

 other Chimaeroids in being open, and protected by cartilaginous 

 rings ; while the skin appears to have been entirely naked. The 

 vertebrae are of the Tectospondylic type of those of the Rays. The 

 skull has been described as hyostylic, but Dr Traquair considers 

 that this is due to crushing, and that it is really of the autostylic 

 structure. 



Family CmM^RiDiE. — Nearly all the remaining forms, from the 

 Lias upwards, may apparently be included in this family, which is 

 now represented by Chimcera and Callorhynchus. The teeth are of 

 enormous size, those of opposite sides meeting in a median sym- 

 physis ; and each tooth has one or more triturating ridges, or pro- 

 minences, differing in appearance from the rest of the tooth, which 

 may be conveniently termed tritors. The type genus Chimcera has 

 the teeth adapted for cutting ; those of the mandible being thin and 

 plate-like, with one large median tritor, and two tritors near the an- 

 terior extremity, and an outer series in the form of dots ; while the 

 palatal tooth varies considerably in shape. This genus is repre- 

 sented at the present day by three species, and has also been re- 

 corded from the Pliocene of Italy and the Miocene of Bavaria. In 

 Elasmodectes {Elasmognathus) of the English Chalk, the mandibular 

 teeth are likewise of a cutting type, but without the median tritor. 

 The extinct Ischyodus, which in England ranges from the Upper 

 Jurassic to the Chalk, but has also been recorded from the Eocene 

 of North America and the Cretaceous of New Zealand, appears to 

 connect Chinmra with the next genus ; the teeth being more adapted 

 for crushing. The mandibular teeth are, indeed, more massive, 

 and generally have two well-marked tritors externally to the large 



