X 
INTRODUCTION 
Lepidotus itself, exhibiting all its typical characters, has also 
been lately discovered in the Upper Trias or Ehaetic ( Prolepidotus 
gcdlineki of Michael). Moreover, it is clear that this genus cannot 
he a specialized form of Colobodus; for all the early species to the 
summit of the Lias exhibit the inner teeth of moderate size on 
comparatively long pedicles, and the powerful tritoral arrangement 
is not observed before the Upper Jurassic. Some of the Wealden 
species are distinguished by the possession of ring-vertebrae; but 
all still retain the thick scales, large biserial fin-fulcra, and com¬ 
plete opercular apparatus. The comparatively regular arrangement 
of this slowly-acquired tritoral dentition is striking, when compared 
with the early approach to a similar type in the Triassic Colobodus. 
The deep-bodied fishes which begin with Dapedius in the Upper 
Trias are noteworthy for the rapid changes exhibited in their form 
and squamation. Cleithrolepis from South Africa and Australia, 
and Tetragonolepis from the European Tapper Lias and the Indian 
Kota Formation, have an excessively deepened trunk, with scales 
strengthened by an unusually thickened anterior margin ; and the 
latter genus is characterized by very thin scales cn the hinder part 
of the caudal region. Still more interesting is the newly-established 
genus Aetheolepis, from the Upper Hawkesbury-Wianamatta Series of 
Kew South Wales, which has the ordinary deepened rhombic scales, 
united by peg-and-socket, on the abdominal region, while these 
gradually lose their articulation, become cycloidal and deeply over¬ 
lapping as they pass backwards towards the pedicle of the tail. 
Among Semionotidse, therefore, a precocious though irregular 
development of grinding-teeth is observed in a genus of the Trias 
(Colobodus ); deep-bodied fishes with styliform rather than tritoral 
teeth, lose their caudal scales in the Upper Lias ( Tetragonolepis ) ; 
and a more normal series (. Lejoidotus ), abundantly represented 
throughout the Jurassic period, retains most of its primitive out¬ 
ward features, while the dentition forms a nearly regular tritoral 
pavement, ring-vertebree are acquired, and the cheek-plates and 
supratemporals are irregularly subdivided in the latest Wealden 
forms. 
Mcicrosemiidce. 
Another series of Protospondyli, with small mouth and styliform 
rather than conical teeth, also appears first in the Trias, and is 
represented throughout the Jurassic period by numerous long¬ 
bodied fishes with extended dorsal fin, while at least one genus 
survives even in the Upper Cretaceous. 
