INTRODUCTION'. 
XV 
until they pass almost imperceptibly into the typical physostomous 
bony fishes; and these forms may be arranged in at least three 
families. 
This classification, adopted in the Catalogue, is a little different 
from any hitherto proposed, because no great systematic value is 
ascribed to the characters of the scales. When one genus of Meso¬ 
zoic ganoid (. Aetheolepis) is proved to exhibit every variety, from the 
thick, rhombic, firmly articulated scales of the abdominal region 
to the delicate, cycloidal, deeply-overlapping scales of the caudal 
pedicle—when, moreover, thick and thin scales cover respectively 
the abdominal and caudal regions of Tetragonolepis, it is no longer 
possible to depend entirely upon scale-characters for the definition 
of families and orders. Since, therefore, Eugnathus and Caturus are 
identical in their osteology, the latter merely differing from the former 
in the subdivision of its supraorbital plates and the tenuity of its 
overlapping scales, these two genera are now placed in one and the 
same family. They and their allies constitute the Eugnathidse, 
which range from the Upper Trias to the Upper Cretaceous; and it 
is difficult to distinguish the more specialized forms from the 
Amiidse. 
Both Eugnatlius and Caturus range from the Upper Trias to the 
Kimmeridgian, and the latter probably even to the Wealden. The 
deeper-bodied Heterolepiclotus, which is scarcely distinguishable 
from Eugnathus , also seems to exhibit as wide a range as the latter. 
More specialized genera, however, like the highly ornate Ptycho- 
lepis (Upper Trias—Upper Lias), the delicate, overgrown Osteo- 
rachis (Lower Lias), and the cycloidal-scaled Eurycormus (Oxfordian 
and Eimmeridgian), are very restricted in their range; so also are 
the Cretaceous genera Neorhombolepis, Otomitla , and Lophiostomus. 
Neorhombolepis and Otomitla are particularly interesting because, 
so far as known, they retain the external characters of Eugnathus , 
and at the same time possess vertebral centra as completely developed 
as those of Amia. If correctly placed in the Eugnathidse, they form 
another illustration of the principle noted among the Semionotidse 
and Macrosemiidse, that the most advanced stage of the endo- 
skeleton is attained in the latest members of the race with the 
least modified exoskeleton. Lophiostomus , which is characterized 
by the fusion of most of its cranial roof-bones, and is provided with 
a pair of bosses on the hinder portion of this roof, seems to exhibit 
only ring-vertebrae. 
