[Illustration: Fig. 71.--Upper Silurian Bivalves. A, _Cardiola interrupta_, Wenlock and Ludlow; B, _Pterinea subfalcata_, Wenlock; C, _Cardiola fibrosa_, Ludlow. (After Salter and M'Coy.)] [Illustration: Fig. 72.--Upper Silurian Gasteropods. a, _Platyceras ventricosum_, Lower Helderberg, America; b, _Euomphalus discors_, Wenlock, Britain; c, _Holopella obsoleta_ Ludlow, Britain; d, _Platyschisma helicites_, Upper Ludlow, Britain; e, _Holopella gracilior_, Wenlock, Britain; f, _Platyceras multisinuatum_, Lower Helderberg, America; g, _Holopea subconica_, Lower Helderberg, America; h, h', _Platyostoma Niagarense_, Niagara Group, America. (After Hall, M'Coy, and Salter.)] [Illustration: Fig 73.--_Tentaculites ornatus_. Upper Silurian of Europe and North America.] The higher groups of the _Mollusca_ are also largely represented in the Upper Silurian. Apart from some singular types, such as the huge and thick-shelled _Megalomi_ of the American Wenlock formation, the Bivalves (_Lamellibranchiata_) present little of special interest; for though sufficiently numerous, they are rarely well preserved, and their true affinities are often uncertain. Amongst the most characteristic genera of this period may be mentioned _Cardiola_ (fig. 71, A and C) and _Pterinea_ (fig. 71, B), though the latter survives to a much later date. The Univalves (_Gasteropoda_) are very numerous, 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 striæ as