The lower groups of the _Mollusca_ are abundantly represented in the marine strata of the Carboniferous series by _Polyzoans_ and _Brachiopods_. Amongst the former, although a variety of other types are known, the majority still belong to the old group of the "Lace-corals" (_Fenestellidoe_), some of the characteristic forms of which are here figured (fig. 126). The graceful netted fronds of _Fenestella, Retepora_, and _Polypora_ (fig. 126, a) are highly characteristic, as are the slender toothed branches of _Glauconome_ (fig. 126, b). A more singular form, however, is the curious _Archimedes_ (fig. 126, c), which is so characteristic of the Carboniferous formation of North America. In this remarkable type, the colony consists of a succession of funnel-shaped fronds, essentially similar to _Fenestella_ in their structure, springing in a continuous spiral from a strong screw-like vertical axis. The outside of the fronds is simply striated; but the branches exhibit on the interior the mouths of the little cells in which the semi-independent beings composing the colony originally lived. [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.)]