202 THE ESSEX NATURALIST.
pectoral (or arm) fins. This arrangement, of course, is very
common among the highest or spine-finned teleosteans (Acanthop-
terygii), but it does not occur in any fishes before the Cretaceous
period, and the Ctenothrissidae, represented in the English Chalk
by Ctenothrissa (fig. 2) and Aulolepis, are the only Clupeoids
known to have acquired it. They are in this respect precocious,
and they seem to have become extinct at the end of the Cre-
taceous period. When first discovered they were not un-
naturally mistaken for spine-finned fishes, and some were
described as belonging to Beryx.
Fig. 2. Ctenothrissa microcephala, Agassiz.
Another peculiar feature also appears for the first time in
some of the teleostean fishes of the Cretaceous period, namely,
the gradual growth backwards of the premaxilla beneath the
maxilla, so that it eventually excludes the latter bone from the
tooth-bearing edge of the upper jaw. This, again, is one of the
special characters of nearly all the Acanthopterygii—an order
which is indeed represented in the Chalk. It occurs, however,
also in some of the lower teleostean fishes of the Chalk, which
seem to include the ancestors of the existing Scopeloids (or
Myctophids, as it is the present prevailing custom to term them).
One of the most interesting of these fishes is Dercetis, of which
a large species (D. maximus) is known by a single fragmentary