198
THE ESSEX NATURALIST.
and Odontaspis, and were referred to these genera by Agassiz.
Some of them may be rightly so identified, but most of the
smaller and more slender teeth of this family belong to a
distinct genus which is represented by complete skeletons
in the Cretaceous of the Lebanon. This shark is especially
remarkable for its elongated shovel-shaped snout, and in 1889
I named it Scapanorhynchus (shovel-snout). The Lamnid sharks
differ from their Jurassic predecessors in a tendency to
acquire a snout strengthened by three prominent cartilages.
It is curious that the extreme development of such a snout
should occur at once in one of the earliest members of the family.
In 1898 Scapanorhynchus (re-named Mitsukurina) was discovered
surviving at the bottom of comparatively deep sea (over 300
fathoms) off Japan, and the living species are now known by
several specimens. Teeth of Scapanorhynchus subulatus occur
in the Chalk at Grays.
Other teeth of Lamnidae common in the Chalk evidently
belong to an extinct genus, and were named Corax by Agassiz.
They are laterally compressed, usually with serrated edges,
and resemble in shape the teeth of the existing Carcharias,
from which they differ in being solid (a Lamnid character). Large
portions of the skeleton have been found associated with the
teeth in the laminated Chalk of Kansas, and the vertebrae are
almost identical with those of the existing Basking Shark,
Selache. It is therefore interesting to remark that the teeth
of Corax affinis, from the uppermost layers of the European
Chalk, are comparatively small and slender, often without
serrations, and much resemble the small teeth of Selache. Corax
may indeed be the ancestor of this Tertiary and Recent genus.
The most characteristic Selachian of the Chalk is Ptychodus,
which seems to have been a large skate. The teeth of several
species and varieties are known, and a peculiar variety of
P. polygyrus is found at Grays. A few associated teeth of this
form are shown in the photographs on Plate xviii., which I owe
to the kindness of the Director of the Geological Survey and
Dr. Kitchin. The teeth are arranged in the jaw in antero-
posterior parallel rows, symmetrical pairs flanking a median
row and gradually becoming smaller outwards. When this
arrangement was first discovered, I supposed that the two
halves of each jaw were in the same straight line, as in the most