XXVlll 
INTRODUCTION. 
dus, the e 5 ’clospon(lylic Palicosjnnaa.’, and the asterosiiondylic Syne- 
chodax ; and an extremely specialized dentition occurs in the existing 
cyclospondylic Spinacidsc, while in many of the asterospondylic 
Scylliidae the teeth have quite a primitive aspect and disposition. 
With regard to the minor grouping of the Selachii into families 
and genera, it is proposed to adopt the arrangement formulated by 
Dr. Gunther in his Catalogue of 1870, for the recent forms ; and 
the various extinct families and genera will he incorporated among 
these in such provisional positions as the available evidence may 
seem to justify. Except in the few instances in which complete 
skeletons are known, this evidence is necessarily very slight, and 
often admits of more than one interpretation ; and no fossils are 
more difficult of satisfactory determination than scattered and iso- 
lated teeth. 
The teeth, in fact, can often be only satisfactorily identified when 
something is known of their geological age and associations. There 
are ITybodont teeth in the Chalk which would be named Orodus if 
found in the Carboniferous ; and some of the teeth of Ithastio age 
might well be mistaken for Cladodns. Lawley also doubtless had a 
prehensile tooth very similar to those of the Cochliodonts when ho 
announced tho discovery of Ilelodm in the ITiocene. But the evi- 
dence of associated remains demonstrates the improbability of all 
these identifications ; and the same kind of evidence must be care- 
fully taken into account when the minor matters of specific nomen- 
clature are under discussion. 
With regard to synonymy, under such circumstances there is 
scope for endless differences of opinion ; and it seems necessary to 
accept as sufficiently defined each name applied to any part of a fish 
precisely and correctly described, which can afterwards be identified 
when satisfactory examples of that particular genus or species happen 
to be discovered. In recent Zoology it is possible to observe a 
stringent rule with reference to complete diagnoses ; but in the 
Paheontology of the Vertebrata, if each investigator is permitted to 
decide whether a recognizably described fossil is sufficient to justify 
the retention or otherwise of the name originally proposed for it, 
generic and specific synonymy will multiply ad wfinitnm. 
In the present state of Palaeontology, it also seems advisable to 
employ generic and specific names in a somewffiat extended sense ; 
and the progress of research will doubtless lead to a further sub- 
division of many of tho groupings now adopted. This Catalogue, 
however, can only be regarded as a provisional attempt to systematize 
