fishes, but the general feeling is that they were an abortive
development in the direction of the fish. The sharks and other large
fishes, which have appeared in the Silurian, easily displace these
clumsy and poor-mouthed competitors One almost thinks of the aeroplane
superseding the navigable balloon.

Of the fishes the Arthrodirans dominated the inland seas (apparently),
while the sharks commanded the ocean. One of the Arthrodirans, the
Dinichthys ("terrible fish"), is the most formidable fish known to
science. It measured twenty feet from snout to tail. Its monstrous head,
three feet in width, was heavily armoured, and, instead of teeth, its
great jaws, two feet in length, were sharpened, and closed over the
victim like a gigantic pair of clippers. The strongly plated heads of
these fishes were commonly a foot or two feet in width. Life in the
waters became more exacting than ever. But the Arthrodirans were
unwieldy and sluggish, and had to give way before more progressive
types. The toothed shark gradually became the lord of the waters.

The early shark ate, amongst other things, quantities of Molluscs and
Brachiopods. Possibly he began with Crustacea; in any case the practice
of crunching shellfish led to a stronger and stronger development of the
hard plate which lined his mouth. The prickles of the plate grew
larger and harder, until--as may be seen to-day in the mouth of a young
shark--the cavity was lined with teeth. In the bulk of the Devonian
sharks these developed into what are significantly called "pavement
teeth." They were solid plates of enamel, an inch or an inch and a half
in width, with which the monster ground its enormous meals of Molluscs,
Crustacea, sea-weed, etc. A new and stimulating element had come into
the life of the invertebrate world. Other sharks snapped larger victims,
and developed the teeth on the edges of their jaws, to the sacrifice
of the others, until we find these teeth in the course of time solid
triangular masses of enamel, four or five inches long, with saw-like
edges. Imagine these terrible mouths--the shears of the Arthrodiran,
and the grindstones and terrible crescents of the giant sharks--moving
speedily amongst the crowded inhabitants of the waters, and it is easy
to see what a stimulus to the attainment of speed and of protective
devices was given to the whole world of the time.

What was the origin of the fish? Here we are in much the same position
as we were in regar