Coming to the _Mollusca_, we note the occurrence of the same great groups as in the Lower Silurian. Amongst the Sea-mosses (_Polyzoa_), we have the ancient Lace-corals (_Fenestella_ and _Retepora_), with the nearly-allied _Glauconome_, and species of _Ptilodictya_ (fig. 66); whilst many forms often referred here may probably have to be transferred to the Corals, just as some so-called Corals will ultimately be removed to the present group. [Illustration: Fig. 66.--Upper Silurian Polyzoa. 1, Fan-shaped frond of _Rhinopora verrucosa_; 1a, Portion of the surface of the same, enlarged; 2 and 2a, _Phoenopora ensiformis_, of the natural size and enlarged; 3 and 3a, _Helopora fragilis_, of the natural size and enlarged; 4 and 4a, _Ptilodictya raripora_, of the natural size and enlarged. The specimens are all from the Clinton Formation (May Hill Group) of Canada. (Original.)] [Illustration: Fig. 67.--_Spirifera hysterica_. The right-hand figure shows the interior of the dorsal valve with the calcareous spires for the support of the arms.] [Illustration: Fig. 68.--Upper Silurian Brachiopods. a a', _Leptocoelia plano-convexa_, Clinton Group, America; b b', _Rhynchonella neglecta_, Clinton Group, America; c, _Rhynchonella cuneata_, Niagara Group, America, and Wenlock Group, Britain; d d', _Orthis elelgantula_, Llandeilo to Ludlow, America and Europe; e e', _Atrypa hemispherica_, Clinton Group, America, and Llandovery and May Hill Groups, Britain; f f', _Atrypa congesta_, Clinton Group, America; g g', _Orthis Davidsoni_, Clinton Group, America. (After Hall, Billings, and the Author.)]