200 THE ESSEX NATURALIST.
to an ancestor of Ptychodus, for a bilaterally-symmetrical tooth
of Hyksobatis, discovered by the late Mr. John S. Gladstone
in the Weald Clay near Cranleigh, Surrey, was worn during life
by three opposing teeth. Ptychodus and Hylaeobatis thus agreed
in having the median tooth of one jaw very small, while the
opposing median tooth was relatively large. I have always
regarded the former jaw as upper, the latter as lower, but a
recent discovery in the Italian Chalk is said to have reversed
my determination.3 Ptychodus disappears at the top of the
English Chalk as suddenly as it appears at the bottom; but
teeth of skates from later Cretaceous and early Eocene forma-
tions in Brazil and West Africa suggest that the genus may
have evolved into some of the modern Myliobatidae or
Trygonidae.
There is little of interest among the Chimaeroid fishes of
the Chalk, which are represented for the most part only by
tooth-plates and fin-spines. They are the survivors and
slightly modified descendants of Jurassic forms, and the last
of the genera, Edaphodon, became extinct in the Miocene period.
I may add that the finest known specimen of the dentition of
Edaphodon sedgwicki, discovered by the late Mr. Charles
Potter in the Chalk of Lewes, Sussex, has lately been acquired
by the Lewes Museum, where it is now accessible for study.
The ganoids are survivals from the Jurassic fauna and
bear marks of racial old age. The very characteristic fringe-
finned Coelacanth, Macropoma, differs from its Jurassic prede-
cessors in the strength of the spines on its scales and in having
lost the fringe of small fin-rays at the end of the tail. Both
these are final specialisations. The sword-fish-shaped Pro-
tosphyraena, of which the long bony snout4 is a familiar fossil
in the Chalk, is merely an extreme form of the Jurassic Pachy-
cormidae, with the powerful teeth sunk into sockets and the
tail unusually powerful. The Pycnodonts show their senility
by the irregular arrangement of their teeth, which are some-
times reduced to cover only part of the supporting bone
(Anomoeodus). The Pycnodonts alone survive beyond the
Chalk, their descendants not becoming extinct until the Upper
Eocene.
3 M. Canavari, Descrizione di un notevole exemplare di Ptychodus Agassiz, Palaeont. Italica,
vol. xxii. (1916), pp. 35-102, pls. v.-xiv.
4 A. S. Woodward, On the Snout of a Pachycormid Fish (Protosphyraena stebbingi) from the
Lower Chalk of S. Ferriby, Lincolnshire, The Naturalist, Nov. 1912, p. 329, pl. xix.