numerous,
but the variety of generic and specific types has now become
greatly reduced. All the branching and complex forms of the Arenig,
the twin-Graptolites and _Dicranograpti_ of the Llandeilo, and
the double-celled _Diplograpti_ and _Climacograpti_ of the Bala
group, have now disappeared. In their place we have the singular
_Retiolites_, with its curiously-reticulated skeleton; and several
species of the single-celled genus _Monograptus_, of which a
characteristic species (_M. Priodon_) is here figured. If we
remove from this group the plant-like _Dictyonemoe_, which are
still present, and which survive into the Devonian, no known
species of _Graptolite_ has hitherto been detected in strata
higher in geological position than the Ludlow. This, therefore,
presents us with the first instance we have as yet met with of
the total disappearance and extinction of a great and important
series of organic forms.

[Illustration: Fig. 58.--A, _Monograptus priodon_, slightly enlarged.
B, Fragment of the same viewed from behind. C, Fragment of the
same viewed in front, showing the mouths of the cellules. D,
Cross-section of the same. From the Wenlock Group (Coniston Flags
of the North of England). (Original.)]

The _Corals_ are very numerously represented in the Upper Silurian
rocks some of the limestones (such as the Wenlock Limestone)
being often largely composed of the skeletons of these animals.
Almost all the known forms of this period belong to the two great
divisions of the Rugose and Tabulate corals, the former being
represented by species of _Zaphrentis, Omphyma, Cystiphyllum,
Strombodes, Acervularia, Cyathophyllum_, &c.; whilst the latter
belong principally to the genera _Favosites, Choetetes, Halysites,
Syringopora, Heliolites_, and _Plasmopora_. Amongst the _Rugosa_, the
first appearance of the great and important genus _Cyathophyllum_,
so characteristic of the Palaeozoic period, is to be noted; and
amongst the _Tabulata_ we have similarly the first appearance,
in force at any rate, of the widely-spread genus _Favosites_--the
"Honeycomb-corals." The "Chain-corals" (_Halysites_), figured
below (fig. 59), are also very common examples of the Tabulate
corals during this period, though they occur likewise in the
Lower Silurian.

[Illustration: Fig. 59.--a, _Halysites catenularia_, small variety,
of the natural size; b, Fragment of a large variety of the same,
of the natural size; c, Fragment of limestone with the t