group,—­one valve being flat and fitting closely into the other, which is more spreading and much fuller.  These, also, were represented by a great variety of species, and we find them crowded together as closely in the ancient rocks as Oysters or Clams or Mussels on any of our modern shores.  Besides these, there were the Bryozoa, a small kind of Mollusk allied to the Clams, and very busy then in the ancient Coral work.  They grew in communities, and the separate individuals are so minute that a Bryozoan stock looks like some delicate moss.  They still have their place among the Reef-Building Corals, but play an insignificant part in comparison with that of their predecessors.

Of the Silurian Univalves or Gasteropods there is not much to tell, for their spiral shells were so brittle that scarcely any perfect specimens are known, though their broken remains are found in such quantities as to show that this class also was very fully represented in the earliest creation.  But the highest class of Mollusks, the Cephalopods or Chambered Shells, or Cuttle-Fishes, as they are called when the animal is unprotected by a shell, are, on the contrary, very well preserved, and they are very numerous.  Of these I will speak somewhat more in detail, because their geological history is a very curious one.

[Illustration]

The Chambered Nautilus is familiar to all, since, from the exquisite beauty of its shell, it is especially sought for by conchologists; but it is nevertheless not so common in our days as the Squids and Cuttle-Fishes, which are the most numerous modern representatives of the class.  In the earliest geological days, on the contrary, those with a shell predominated, differing from the later ones, however, in having the shell perfectly straight instead of curved, though its internal structure was the same as it is now and has ever been.  Then, as now, the animal shut himself out from his last year’s home, building his annual wall behind him, till his whole shell was divided into successive chambers, all of which were connected by a siphon.  Some of the shells of this kind belonging to the Silurian deposits are enormous:  giants of the sea they must have been in those days.  They have been found fifteen feet long, and as large round as a man’s body.  One can imagine that the Cuttle-Fish inhabiting such a shell must have been a formidable animal.  These straight-chambered shells of the Silurian and Devonian seas are called Orthoceratites (see wood-cut below).  We shall meet them again hereafter, under another name and with a different form; for, as they advance in the geological ages, they not only assume the curved outline with ever closer whorls till it culminates in the compact coil of the Ammonites of the middle periods, but the partitions, which are perfectly plain walls in these earlier forms, become scalloped and involuted along the edges in the later ones, making the most delicate and exquisite tracery on the surface of the shell.