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.)]

The Brachiopods continued to flourish during the Upper Silurian Period in immense numbers and under a greatly increased variety of forms.  The three prominent Lower Silurian genera Orthis, Strophomena, and Leptoena are still well represented, though they have lost their former preeminence.  Amongst the numerous types which have now come upon the scene for the first time, or which have now a special development, are Spirifera and Pentamerus.  In the first of these (fig. 69. b, c), one of the valves of the shell (the dorsal) is furnished in its interior with a pair of great calcareous spires, which served for the support of the long and fringed fleshy processes or “arms” which were attached to the sides of the mouth.[16] In the genus Pentamerus (fig. 70) the shell is curiously subdivided in its interior by calcareous plates.  The Pentameri commenced their existence at the very close of the Lower Silurian (Llandovery), and survived to the close of the Upper Silurian; but they are specially characteristic of the May Hill and Wenlock groups, both in Britain and in other regions.  One species, Pentamerus galeatus, is common to Sweden, Britain, and America.  Amongst the remaining Upper Silurian Brachiopods are the extraordinary Trimerellids; the old and at the same time modern Linguloe, Discinoe, and Cranioe; together with many species of Atrypa (fig. 68, e), Leptocoelia (fig. 68, a), Rhynchonella (fig. 68, b, c), Meristella (fig. 69, a, e, f), Athyris, Retzia, Chonetes, &c.