in Britain, France, Spain, Belgium, Germany, Russia, and China.  The family of the Productidoe commenced to exist in the Upper Silurian, in the genus Chonetes, and we shall hereafter find it culminating in the Carboniferous in many forms of the great genus Producta[17] itself.  In the Devonian period, there is an intermediate state of things, the genus Chonetes being continued in new and varied types, and the Carboniferous Produdoe being represented by many forms of the allied group Productella.  Amongst other well-known Devonian Brachiopods may be mentioned the two long-lived and persistent types Atrypa reticularis (fig. 97) and Strophomena rhomboidalis (fig. 98).  The former of these commences in the Upper Silurian, but is more abundantly developed in the Devonian, having a geographical range that is nothing less than world-wide; whilst the latter commences in the Lower Silurian, and, with an almost equally cosmopolitan range, survives into the Carboniferous period.

[Footnote 17:  The name of this genus is often written Productus, just as Spirifera is often given in the masculine gender as Spirifer (the name originally given to it).  The masculine termination to these names is, however, grammatically incorrect, as the feminine noun cochlea (shell) is in these cases understood.]

[Illustration:  Fig. 98.—­Strophomena rhomboidalis.  Lower Silurian, Upper Silurian, and Devonian of Europe and America.]

[Illustration:  Fig. 99.—­Different views of Platyceras dumosum, of the natural size.  Devonian, Canada. (Original.)]

The Bivalves (Lamellibranchiata) of the Devonian call for no special comment, the genera Pterinea and Megalodon being, perhaps, the most noticeable.  The Univalves (Gasteropods), also, need not be discussed in detail, though many interesting forms of this group are known.  The type most abundantly represented, especially in America, is Platyceras (fig. 99), comprising thin, wide-mouthed shells, probably most nearly allied to the existing “Bonnet-limpets,” and sometimes attaining very considerable dimensions.  We may also note the continuance of the genus Euomphalus, with its discoidal spiral shell.  Amongst the Heteropods, the survival of Bellerophon is to be recorded; and in the “Winged-snails,” or Pteropods, we find new forms of the old genera Tentaculites and Conularia (fig. 100).  The latter, with its fragile, conical, and often beautifully ornamented shell, is especially noticeable.

[Illustration:  Fig. 100.—­Conularia ornata, of the natural size.  Devonian, Europe.]

[Illustration:  Fig. 101.—­Clymenia Sedgwickii.  Devonian, Europe.]