INTRODUCTION. 
Vll 
the base of the skull. This upper dentition is henceforth usually 
opposed not to the mandible but to a dental arrangement on tho 
tongue or hyoid apparatus. 
One of the common Cretaceous fishes ( Istieus , p 00), here placed 
in the Albulid®, is interesting as being essentially identical with 
an imperfectly-known fish still surviving in the deep sea ( Bathij - 
thrissa). It is the most elongated typo of this primitive group, 
with multiplied vertebrae and an extended dorsal fin. 
The Ostooglossidae, with a curiously-thickened skull, also seem 
to be closely related to the early Albulid*. The fragmentary fossils 
already discovered suggest that these families will need a new 
definition as soon as their extinct representatives are satisfactorily 
known. 
The primitive nature of the Chirocontridoe has long been inferred 
from the presence of a rudimentary spiral valve in the intestine of 
the sole surviving species, Ohirocentrus dorab l . This family of 
fishes is, indeed, now proved to be very old, dating back at least to 
the beginning of the Crotaceous period, during which it attainod its 
maximum development. Early Cretaceous forms, such as Ohiro- 
centrites, cannot even be distinguished from the typical species of the 
Upper Jurassic Thrissops until the cranium be available for detailed 
study. It is still not definitely determined that Thrissops belongs 
to the lower division (family Leptolepidse) in which it is usually 
placed. 
The true Clupeoid fishes also date back to the beginning of the 
Cretaceous period, and their skeleton is so closely similar to that of 
the typical Jurassio Leptolepidae that they may well be direct 
descendants of the latter. Their higher grade is definitely indicated 
by the prominence of the supraoecipital and otic bones in the skull. 
Most of the Cretaceous forms are typical Clupeidse, and they have 
scarcely changed during subsequent epochs. A few, however, dis- 
covered only in Cretaceous rocks, are of special interest as exhibiting 
the precocious development of a character which was never per- 
manently acquired by fishes with so primitive a skull, but soon 
became the common feature of the spiny-finned or acanthopterygian 
families. These are the Ctenothrissidic, which hare hitherto been 
mistaken for Berycoids because they display tho character in 
question, namely, the forward displacement of their pelvic fins, 
which are situated more or less directly beneath the pectoral pair. 
1 Cuvier and Valenciennes, ‘Histoire Naturelle des Poissons,’ vol. xix. 
(1816), p. 160, pi. 565. 
