[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 the supporting loops or spires, which serve as one of the special characters distinguishing the Brachiopods from the true Bivalves (Lamellibranchiata).]

[Illustration:  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.)]

[Illustration:  Fig. 70.—­Pentamerus Knightii.  Wenlock and Ludlow.  The right-hand figure shows the internal partitions of the shell.]

[Illustration:  Fig. 71.—­Upper Silurian Bivalves.  A, Cardiola interrupta, Wenlock and Ludlow; B, Pterinea subfalcata, Wenlock; C, Cardiola fibrosa, Ludlow. (After Salter and M’Coy.)]

[Illustration:  Fig. 72.—­Upper Silurian Gasteropods. a, Platyceras ventricosum, Lower Helderberg, America; b, Euomphalus discors, Wenlock, Britain; c, Holopella obsoleta Ludlow, Britain; d, Platyschisma helicites, Upper Ludlow, Britain; e, Holopella gracilior, Wenlock, Britain; f, Platyceras multisinuatum, Lower Helderberg, America; g, Holopea subconica, Lower Helderberg, America; h, h’, Platyostoma Niagarense, Niagara Group, America.  (After Hall, M’Coy, and Salter.)]

[Illustration:  Fig 73.—­Tentaculites ornatus.  Upper Silurian of Europe and North America.]

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 special interest; for though sufficiently numerous, they are rarely well preserved, and their true affinities are often uncertain.  Amongst the most characteristic genera of this period may be mentioned Cardiola (fig. 71, A and C) and Pterinea (fig. 71, B), though the latter survives to a much later date.  The Univalves (Gasteropoda) are very numerous,