Fig. 48.—Ptilodictya falciformis. a, Small specimen of the natural size; b, Cross-section, showing the shape of the frond; c, Portion of the surface, enlarged. Trenton Limestone and Cincinnati Group, America. (Original.)
Fig. 49.—A, Ptilodictya acuta; B. Ptilodictya Schafferi. a, Fragment, of the natural size; b, Portion, enlarged to show the cells. Cincinnati Group of Ohio and Canada. (Original.) different forms of Brachiopods which present themselves in the Lower Silurian deposits; but we may select the three genera Orthis, Strophomena, and Leptœna for illustration, as being specially characteristic of this period,
Fig. 50.—Lower Silurian Brachiopods. a and a', Orthis biforata, Llandeilo-Caradoc, Britain and America: b, Orthis flabellulum, Caradoc, Britain: c, Orthis subquadrata, Cincinnati Group, America; c', Interior of the dorsal valve of the same: d, Strophomena deltoidea, Llandeilo-Caradoc, Britain and America. (After Meek, Hall, and Salter.) though not exclusively confined to it. The numerous shells which belong to the extensive and cosmopolitan genus Orthis (fig. 50, a, b, c, and fig. 51, c and d), are usually more or less transversely-oblong or subquadrate, the two valves (as more or less in all the Brachiopods) of unequal sizes,
Fig. 51.—Lower Silurian Brachiopods, a, Strophomena alternata, Cincinnati Group, America; b, Strophomena filitexta, Trenton and Cincinnati Groups, America; c, Orthis testudinaria, Caradoc, Europe, and America; d, d', Orthis plicateila, Cincinnati Group, America; e, e', e'', Leptœna sericea, Llandeilo and Caradoc, Europe and America. (After Meek, Hall, and the Author.) generally more or less convex, and marked with radiating ribs or lines. The valves of the shell are united to one another by teeth and sockets, and there is a straight hinge-line. The beaks are also separated by a distinct space ("hinge-area"), formed in part by each valve, which is perforated by a triangular opening, through which, in the living condition, passed a muscular cord attaching the shell to some foreign object. The genus Strophomena (fig. 50, d, and 51, a and b) is very like Orthis in general character; but the shell is usually much flatter, one or other valve often being concave, the hinge-line is longer, and the aperture for the emission of the stalk of attachment is partially closed by a calcareous plate. In Leptœna, again (fig. 51, e), the shell is like Strophomena in many respects, but generally comparatively longer, often completely semicircular, and having one valve convex and the other valve concave. Amongst other genera of Brachiopods which are largely represented in the Lower Silurian rocks may be mentioned Lingula, Crania, Discina, Trematis, Siphonotreta, Acrotreta, Rhynchonella, and Athyris; but none of these can claim the importance to which the three previously-mentioned groups are entitled.
The remaining Lower Silurian groups of Mollusca can be but briefly glanced at here. The Bivalves (Lamellibranchiata) find numerous representatives, belonging to such genera as Modiolopsis, Ctenodonta, Orthonota, Palœarca, Lyrodesma,
Fig. 52.—Murchisonia gracilis, Trenton Limestone, America. (After Billings.) Ambonychia,and Cleidophorus. The Univalves (Gasteropoda) are also very numerous, the two most important genera being Murchisonia (fig. 52) and Pleurotomaria. In both these groups the outer lip of the shell is notched; but the shell in the former is elongated and turreted, whilst in the latter it is depressed. The curious oceanic Univalves known as the Heteropods are also very abundant, the principal forms belonging to Bellerophon and Maclurea. In the former (fig. 53) there is a symmetrical convoluted shell, like that of the Pearly Nautilus in shape, but without any internal partitions, and having the aperture often expanded and notched behind. The species of Maclurea (fig. 54) are found both in North America and in Scotland, and are exclusively confined to the Lower Silurian period, so far as known. They have the shell coiled into a flat spiral, the mouth being furnished with a very curious, thick, and solid lid or "operculum." The Lower Silurian Pteropods, or "Winged snails," are numerous, and belong principally to the genera Theca, Conularia, and Tentaculites, the last-mentioned of these often being extremely abundant in certain strata.