and a few characteristic forms are here figured (fig. 72).  Of these, no genus is perhaps more characteristic than Euomphalus (fig. 72, b), with its flat discoidal shell, coiled up into an oblique spiral, and deeply hollowed out on one side; but examples of this group are both of older and of more modern date.  Another very extensive genus, especially in America, is Platyceras (fig. 72, a and f), with its thin fragile shell—­often hardly coiled up at all—­its minute spire, and its widely-expanded, often sinuated mouth.  The British Acroculioe should probably be placed here, and the group has with reason been regarded as allied to the Violet-snails (Ianthina) of the open Atlantic.  The species of Platyostoma (fig. 72, h) also belong to the same family; and the entire group is continued throughout the Devonian into the Carboniferous.  Amongst other well-known Upper Silurian Gasteropods are species of the genera Holopea (fig. 72, g), Holopella (fig. 72. e), Platyschisma (fig. 72, d), Cyclonema, Pleurotomaria, Murchisonia, Trochonema, &c.  The oceanic Univalves (Heteropods) are represented mainly by species of Bellerophon; and the Winged Snails, or Pteropods, can still boast of the gigantic Thecoe and Conularioe, which characterise yet older deposits.  The commonest genus of Pteropoda, however, is Tentaculites (fig. 73), which clearly belongs here, though it has commonly been regarded as the tube of an Annelide.  The shell in this group is a conical tube, usually adorned with prominent transverse rings, and often with finer transverse or longitudinal striae as well; and many beds of the Upper Silurian exhibit myriads of such tubes scattered promiscuously over their surfaces.

The last and highest group of the Mollusca—­that of the Cephalopoda—­is still represented only by Tetrabranchiate forms; but the abundance and variety of these is almost beyond belief.  Many hundreds of different species are known, chiefly belonging to the straight Orthoceratites, but the slightly-curved Cyrtoceras is only little less common.  There are also numerous forms of the genera Phragmoceras, Ascoceras, Gyroteras, Lituites, and Nautilus.  Here, also, are the first-known species of the genus Goniatites—­a group which attains considerable importance in later deposits, and which is to be regarded as the precursor of the Ammonites of the Secondary period.

[Illustration:  Fig. 74.—­Head-shield of Pteraspis Banksii, Ludlow rocks. (After Murchison.)]

[Illustration:  Fig. 75.—­A, Spine of Onchus tenuistriatus; B, Shagreen-scales of Thelodus.  Both from the “bone-bed” of the Upper Ludlow rocks. (After Murchison.)]