Fossil Anthozoa.--The first group of extinct corals to be noticed under this head is the _Graptolitidæ_, a family restricted to the Silurian rocks, and whose natural affinities have been much questioned; some palæontologists referring them to the _Pennatulidæ_, or Sea-pens, others to the _Sertulariadæ_.[232]
[232] For a full consideration of this subject, refer to Prof. McCoy's Brit. Palæozoic Fossils.
[Illustration: Lign. 85. Graptolites in Wenlock Limestone.
Fig. 1.--Graptolites ludensis.[233] 1_a_.--Magnified view of a portion of the same. 2.--Graptolites Murchisoni. 2_a_.--Magnified portion of _fig._ 2. ]
[233] _Ludensis_, from Ludlow--to indicate the habitat of the fossils.
Graptolites. _Lign. 85._--These curious zoophytes abound in many of the Silurian deposits; they consist of sessile polype cells, arranged in one or two rows to a flexible stem, like the recent Sertularia, or Virgularia. Prof. McCoy refers them to the order Hydroida.
In a recent state these bodies were probably covered with a soft, or albuminous mass, studded with polype-cells, disposed in rows along the margins of the lateral, curved, grapple-like processes, as in the zoophytes termed _Virgularia_,[234] to which one kind bears a great analogy. If two specimens of the _Graptolites Ludensis_ be placed together, so that the elongated smooth edges be in apposition, the united stems will be seen to offer a general resemblance to the axis of _Virgularia mirabilis_.
M. Barrande divides the Graptolites into three groups or genera, which are defined as follow:--
_Graptolites_ (proper), a single series of cells united together at the base, and adhering along the sides nearly to the orifice of each cell, as in _fig. 1a_. _Monoprion_ of M. Barrande.
_Rastrites._--The axis reduced to a mere line, on which the cells are placed at relatively wide intervals, and but slightly inclined. These two genera are supposed to have been hydroid zoophytes, and related to the Sertularidæ.
_Diprion_ (_Diplograpsus_ of Mr. McCoy), cells in two series arranged along a central axis; these forms present a foliaceous appearance; they are presumed to resemble the existing genera Pennatula and Virgularia.
Graptolites have been found in strata of the same age in Norway, Sweden, and Scotland.[235] I have received slates literally covered with them, from the Silurian rocks of the United States, by the kindness of my friend, Benjamin Silliman, jun. Esq.