206
THE ESSEX NATURALIST.
which have retreated to the deep sea have assumed fantastic
shapes. It seems not unlikely that the eels originated directly
from some of the long-bodied ganoids, and it is worthy of note
that one of the Upper Jurassic Macrosemiidae, Enchelyolepis,
is covered with very thin scales which, as vaguely seen in the
fossils, appear much like the scales of Anguilla.9
From this brief summary it is evident that in the open sea
of the Cretaceous period, a completely new era in the evolution
of fishes was inaugurated. The changes in the back of the skull
and the margin of the upper jaw, the development of spines on
the opercular bones and cheek, the hardening of some fin-rays
into spines, and the movable situation of the pelvic fins, led to
the possibility of almost infinite variety in shape and adaptation.
Immediately after the Cretaceous period the predominant fishes
assumed more multifarious forms than they had ever done
before, and they penetrated for the first time the inhospitable
depths of the ocean. It was precisely at the same period that
the Age of Reptiles came to an end and the warm-blooded
mammals and birds suddenly spread and multiplied. World-
revolutions of various kinds have sometimes been invoked to
account for the latter phenomenon, but such explanations can
scarcely apply to the rapid culmination in the development of
the teleostean fishes in the ocean. This affords a problem that
remains to be solved.
EXPLANATION OF PLATE XVIII.
Ptychodus polygyrus, Agassiz; group of associated teeth, nat. size.—
Upper Chalk; Grays, Essex. Photograph of specimen in the Museum
of Practical Geology by Mr. J. Rhodes, jun., reproduced by permission of
the Director of the Geological Survey.
Fig. 1.—Tooth probably of first inner paired row of upper jaw.
Fig. 2.—Three teeth of paired lateral rows probably of upper jaw,
with part of a small median upper tooth on the extreme
left.
EXPLANATION OF THE TEXT FIGURES.
Fig. 1.—Mandible of Ptychodus decurrens, Agassiz, from the Lower
Chalk of Sussex; reduced.
Fig. 2.—Restoration of Ctenothrissa microcephala, Agassiz, from the
Lower Chalk; reduced.
Fig. 3.—Restored skeleton of Leptotrachelus triqueter, Pictet, and photo-
graph of specimen from the Upper Cretaceous, Sahel Alma,
Mt. Lebanon, showing swallowed fish in distended stomach;
much reduced.
9 A. S. Woodward, Fossil Fishes of the English Wealden and Purbeck Formations, Mon.
Palaeont. Soc. 1918, p. 81, pl. xvii., fig. 6b.