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 Leptœna 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
Fig. 66.—Upper Silurian Polyzoa. 1, Fan-shaped frond of Rhinopora verrucosa; 1a, Portion of the surface of the same, enlarged; 2 and 2a, Phœnopora 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.) 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 Lingulœ, Discinœ, and Craniœ; together with many species of Atrypa (fig. 68, e),
Fig. 68.—Upper Silurian Brachiopods. a a', Leptocœlia 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.) Leptocœlia (fig. 68, a), Rhynchonella (fig. 68, b, c), Meristella (fig. 69, a, e, f), Athyris, Retzia, Chonetes, &c.
[Footnote 16: In all the Lamp-shells the mouth is provided with two long fleshy organs, which carry delicate filaments on their sides, and which are usually coiled into a spiral. These organs are known as the "arms," and it is from their presence that the name of "Brachiopoda" is derived (Gr. brachion, arm; podes, feet). In some cases the arms are merely coiled away within the shell, without any support; but in other cases they are carried upon a more or less elaborate shelly loop, often spoken of as the "carriage-spring apparatus." In the Spirifers, and in other ancient genera, this apparatus is coiled up into a complicated spiral (fig. 67). It is these "arms," with or without
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. the supporting loops or spires, which serve as one of the special characters distinguishing the Brachiopods from the true Bivalves (Lamellibranchiata).]
| Fig. 69.—a a', Meristella intermedia, Niagara Group, America; b, Spirifera Niagarensis, Niagara Group, America; c c', Spirifera crispa, May Hill to Ludlow, Britain, and Niagara Group, America; d, Strophomena (Streptorhynchus) subplana, Niagara Group, America; e, Meristella naviformis, Niagara Group, America; f, Meristella cylindrica, Niagara Group, America. (After Hall, Billings, and the Author.) |
The higher groups of the Mollusca are also largely represented in the Upper Silurian. Apart from some singular types, such as the huge and thick-shelled Megalomi of the American Wenlock formation, the Bivalves (Lamellibranchiata) present little of