[Illustration:  Fig. 127.—­Carboniferous Braciopoda. a, Producta semireticulata_, showing the slightly concave dorsal valve; a’ Side view of the same, showing the convex ventral valve; b, Producta longispina; c, Orthis resupinata; d, Terebratula hastata; e, Athyris subtilita; f, Chonetes Hardrensis; g, Rhynchonella pleurodon; h, Spirifera trigonalis.  Most of these forms are widely distributed in the Carboniferous Limestone of Britain, Europe, America, &c.  All the figures are of the natural size. (After Davidson, De Koninck, and Meek.)]

The Brachiopods are extremely abundant, and for the most part belong to types which are exclusively or principally Palaeozoic in their range.  The old genera Strophomena, Orthis (fig. 127, c), Athyris (fig. 127, e), Rhynchonella (fig. 127, g), and Spirifera (fig. 127, h), are still well represented—­the latter, in particular, existing under numerous specific forms, conspicuous by their abundance and sometimes by their size.  Along with these ancient groups, we have representatives—­for the first time in any plenty—­of the great genus Terebratula (fig. 127, d), which underwent a great expansion during later periods, and still exists at the present day.  The most characteristic Carboniferous Brachiopods, however, belong to the family of the Productidoe, of which the principal genus is Producta itself.  This family commenced its existence in the Upper Silurian with the genus Chonetes, distinguished by its spinose hinge-margin.  This genus lived through the Devonian, and flourished in the Carboniferous (fig. 127, f).  The genus Producta itself, represented in the Devonian by the nearly allied Productella, appeared first in the Carboniferous, at any rate, in force, and survived into the Permian; but no member of this extensive family has yet been shown to have over-lived the Palaeozoic period.  The Productoe of the Carboniferous are not only exceedingly abundant, but they have in many instances a most extensive geographical range, and some species attain what may fairly be considered-gigantic dimensions.  The shell (fig. 127, a and b) is generally more or less semicircular, with a straight hinge-margin, and having its lateral angles produced into larger or smaller ears (hence its generic name—­“cochlea producta").  One valve (the ventral) is usually strongly convex, whilst the other (the dorsal) is flat or concave, the surface of both being adorned with radiating ribs, and with hollow tubular spines, often of great length.  The valves are not locked together by teeth, and there is no sign in the fully-grown shell of an opening in or between the valves for the emission of a muscular stalk for the attachment of the shell to foreign objects.  It is probable, therefore, that the Productoe, unlike the ordinary Lamp-shells, lived an independent existence, their long spines apparently serving to