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.

[Illustration:  Fig. 52.—­Murchisonia gracilis, Trenton Limestone, America. (After Billings.)]

[Illustration:  Fig. 53.—­Different views of Bellerophon Argo, Trenton Limestone, Canada. (After Billings.)]

[Illustration:  Fig. 54.—­Different views of Maclurea crenulata, Quebec Group, Newfoundland. (After Billings.)]

[Illustration:  Fig. 55.—­Fragment of Orthoceras crebriseptum, Cincinnati Group, North America, of the natural size.  The lower figure section showing the air-chambers, and the form and position of the siphuncle. (After Billings.)]

[Illustration:  Fig. 56.—­[14] Restoration of Orthoceras, the shell being supposed to be divided vertically, and only its upper part being shown. a, Arms; f, Muscular tube ("funnel”) by which water is expelled from the mantle-chamber; c, Air-chambers; s, Siphuncle.]

[Footnote 14:  This illustration is taken from a rough sketch made by the author many years ago, but he is unable to say from what original source it was copied.]

Lastly, the Lower Silurian Rocks have yielded a vast number of chambered shells, referable to animals which belong to the same great division as the Cuttle-fishes (the Cephalopoda), and of which the Pearly Nautilus is the only living representative at the present day.  In this group of Cephalopods the animal possesses a well-developed external shell, which is divided into chambers by shelly partitions ("septa").  The animal lives in the last-formed and largest chamber of the shell, to which it is organically connected by muscular attachments.  The head is furnished with long muscular processes or “arms,” and can be protruded from the mouth of the shell at will, or again withdrawn within it.  We learn, also, from the Pearly Nautilus, that these animals must have possessed two pairs of breathing organs or “gills;” hence all these forms are grouped