[Illustration: FIG. 61.—Group of Older Palæozoic Lamellibranchs.—After Billings. 1, _Cucullea opima_. 2, _Nucula oblonga_. 3, _Nucula lineata_. 4, _Cypricardia truncata_. 5, _Tellina ovata_. 6, _Nucula bellatula_. 7, _Modiola concentrica_.] The ordinary bivalves, like the mussels and cockles, now so very plentiful on our coasts, are rare in the Cambrian and Silurian, and for the first time make a somewhat conspicuous appearance in the Upper Silurian and Devonian. But from the first they resemble very closely their modern successors, though on the whole neither so large nor so ornate (Fig. 61). Their fortunes have thus been precisely the opposite of those of the Brachiopods, though in neither case is there very marked elevation or deterioration in the individual animals. A very similar statement may be made as to the sea-snails, whether the curious winged snails (Pteropods) or the ordinary crawlers (Gastropods). The former come in early, and are represented by Palæozoic forms finer than any now extant. The genus _Conularia_ (Fig. 62) presents some Silurian species six inches or more in length, which are giants in comparison with any now living. The forms of more ordinary Gastropods from the Silurian represented in Fig. 63 will suffice to show that their styles are not very dissimilar from those still extant.[17] As in the case of the ordinary bivalves, however, the modern Gastropods much exceed in numbers and magnitude those of the Palæozoic. [Illustration: FIG. 62.—_Conularia planicostata_ (Dn.). A Carboniferous Pteropod.] [Illustration: FIG. 63.—Silurian Sea-snails. Canada. _a_, _Murchisonia bicincta_ (Hall). _b_, _Pleurotomaria umbilicatula_ (Hall). _c_, _Murchisonia gracilis_ (Hall). _d_, _Bellerophon sulcatinus_ (Billings).] The highest group of Mollusks, represented in the modern ocean by the Nautili and Cuttle-fishes, has a history so strange and eventful, and so different from what might have been anticipated, that it perhaps deserves a more detailed notice, more especially as Barrande has recently directed marked attention to it in his magnificent work on the Palæontology of Bohemia.