THE FOSSIL FISHES OF THE CHALK.
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The lowest of the Chalk teleosteans are the Clupeoids, Osteo-
glossids, and Chirocentrids—herrings and their allies. They
differ little from some of the Jurassic Leptolepidae and
Pholidophoridae, and may have been derived from unknown
members of these or related families. Many of them have
the primitive form of the back of the head, with the parietal
bones in contact and the squamosals completely covering the
bones surrounding the ear (an arrangement characteristic of
almost all the Jurassic ganoids and still found in the existing
teleosteans Elops, Megalops and Albula); while others agree
with the true herrings and Chirocentrids in being modernised,
the supra-occipital and the bony ear-region projecting between
and beyond the reduced parietals and squamosals. When their
skulls are studied in detail, however, it becomes evident that
the Cretaceous groups or families were not so clearly separated
as are their descendants at the present day. They may,
therefore, well be regarded as the generalised ancestors of the
living forms. Most of these still live in shallow seas and near
the surface of the open ocean, but a few have retreated to oceanic
depths (such as the Alepocephalidae), while others are found
in freshwaters (such as the typical Salmonidae and the
Osteoglossidae). Pachyrhizodus and Osmeroides may be
mentioned as the commonest of the primitive genera in the
Chalk. Portheus, known only by fragments in our English
Chalk, but by complete specimens 12 feet long in the Chalk of
Kansas, U.S.A., is the largest Upper Cretaceous bony fish hitherto
discovered, and is very closely related to Chirocentrus, which is
the sole survivor of the group now restricted to the Indian
Ocean. Ichthyodectes and Saurodon are represented by smaller
species in the Chalk. Plethodus and Anogmius,5 with a great
crushing tooth-plate on the palate opposed to another on the
hyoid bones, are probably related to the existing freshwater
Osteoglossidae. Other fishes from the Chalk, known only by
clusters of small crushing teeth on plates of bone, were probably
nearly similar to Albula, which is the solitary representative of
its family still living in tropical and sub-tropical seas.
At least one family of Clupeoids in the Chalk is remarkable
as having the pelvic (or leg) fins fixed far forwards beneath the
5 A. S. Woodward, On a new Fossil Fish (Anogmius ornatus, sp. nov.), from the Lower Chalk
of South Ferriby, Lincolnshire, The Naturalist, Sept. 1923, p. 297.