Till 
IN-TEODTJCTION-. 
The few undivided rays in front of their fins, however, are always 
articulated distally and never form true spines. 
Among the typical Cretaceous Clupeidse the early acquisition of 
the ventral ridge-scutes, peculiar to Clupea and its allies, is specially 
noteworthy. Some small fishes from the Neocomianof Switzerland 
(p. 138) exhibit these scutes quite clearly. A widespread marine 
Cretaceous genus, Diplomystus^ possessed dorsal ridge-scutes as well 
as the ventral series. It became a freshwater fish in the Eocene 
period, and now survives only in the rivers of New South Wales 
and Chili. Pectinated scales also characterised one Cretaceous 
genus {Fseiidoberyx), which is proved to be a Clupeoid—and not a 
Berycoid as hitherto supposed—by the distinctly observable structure 
of the head and fins. 
Nothing is definitely known concerning the extinct Salmonoids ; 
and Palaeontology affords no information concerning the origin or 
relationships of this family. 
Of all Cretaceous Isospondyli, three families of eel-shaped fishes 
are the most difficult to understand. They are all characterised by 
a primitive cranium of the Jurassic type; but they exhibit the 
new specialisation by which the extending premaxiUa gradually 
excludes the maxilla from the upper border of the mouth. Their 
elongated shape is alone indicative of high specialisation ; but no 
intermediate forms are yet known to afford a clue to their more 
normally shaped ancestors. Of these families the Halosauridse and 
Notacanthidae still survive in the deep sea. The existing Halo- 
saurus itself, indeed, cannot be clearly distinguished from the 
Cretaceous EcMdnocephalus ; while Notacanihus of the present 
fauna only seems to differ from Pronotacanthus of the Cretaceous 
period in the possession of dorsal spines and fin-rays. The Der- 
cetidag, on the other hand, are only known by fossils from Cre¬ 
taceous formations, in which they are widely distributed. They 
are interesting as being the earliest type of fish in which evidence 
of a distensible stomach has been observed (p. 177). Their fins 
are less specialised than those of the two families just mentioned ; 
and their trunk is provided with paired longitudinal series of 
enlarged scutes. 
Isospondyli exhibiting a definite tendency towards the develop¬ 
ment of acanthopterygian fishes seem to have been almost as 
abundant in Cretaceous seas as the Elopines and Clupeoids. In 
fact, in the absence of a complete knowledge of the skeleton, some 
of the Cretaceous fishes might be referred equally well to the 
