IXTKODTTCTIOX. 
IX 
full development. It is true that there are links between most of 
the families, rendering precise definitions almost impossible; and 
evidences of evolution can he detected in a slight degree as the 
different groups are traced upwards in their range. All the 
families, however, except the modern Lepidosteidae and Amiidae, 
had already become differentiated before the period of the Lower 
Oolites. 
Semionotidce. 
Kobust Protospondyli with a small mouth and grinding teeth, 
and predaceous forms with a large mouth and conical teeth, appear 
abundantly in the Trias; hut as the former do not seem to pass 
into modern honv fishes, while the latter can he distinctly traced 
towards the characteristic types of Upper Cretaceous and Tertiary 
times, those with a small mouth and more or less tritoral dentition 
are treated first. 
This arrangement seems all the more natural since the family 
of Semionotidae—the most generalized of the series—is first re¬ 
presented by Acentrophorus in the Upper Permian. Ho other 
representative of the Protospondyli has hitherto been discovered 
in Palaeozoic formations, and it is interesting to notice that this 
unique fish is the most generalized genus of the family to which it 
belongs. Before the end of the Triassic period, however, all the 
principal types of Semionotidae had appeared; and the most specialized 
genus Tetragonolepis occurs in the Upper Lias. 
The Triassic genera most closely related to Acentrophorus , 
namely, Semionotus and Pristisomus , scarcely differ from it except 
in possessing well-developed ridge-scales. The former is confined to 
the Trias in Horth America, and is not known to range above the 
Bheetic in Europe. There is, however, an interesting allied genus, 
Aphnelepis, in the supposed Jurassic of Hew South Wales; and 
this exhibits comparatively thin scales, which suddenly become 
extremely delicate on the caudal region behind a line connecting 
the points of origin of the dorsal and anal fins. 
The Triassic genus Colobodus scarcely differs from Semionotus 
except in its powerful grinding dentition and the reduction of its 
fin-fulcra to a fine and close series. In many respects it resembles 
Lepidotus : hut the tooth-pavement does not appear to exhibit any 
regular arrangement, and the scale-articulation is not strengthened 
by the developtnent of spurs from the overlapped margin such as 
characterize even the earliest species of Lepidotus. 
