THE UPPER SILURSAN, PERIOD. 
Ii9 
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-Grap- 
tolites and Duzcranograpti of the 
Llandeilo, and the double-celled 
Diplograptt 
and Clhmacograpti of 
the Bala group, have now disap- 
peared. 
In their place we have 
the singular Refzolzes, with its curi- 
ously-reticulated skeleton; and seve- 
ral species of the single-celled genus 
Monograptus, of which a character- 
‘istic species (JZ. priodon) is here 
figured. 
If we remove from this 
group the plant-like Ductyoneme, 
which are still present, and which 
survive 
into 
the Devonian, no 
known species of Gvaftolite has 
hitherto been detected 
in strata 
higher in geological position than 
the Ludlow. 
This, therefore, pre- 
sents us with the first instance we 
have as yet met with of the total 
disappearance and extinction of a 
great and important series of or- 
ganic forms. 
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, Cyathophylum, 
Fig. 58.—A, IMWonograptus prio- 
don, slightly enlarged. B, Frag- 
ment 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.) 
SC 
whilst the latter belong principally to the genera Savosites, 
Chetetes, Halysites, Syringopora, Heliolites, and Plasmopora. 
Amongst the Awgosa, the first appearance of the great and 
important genus Cyathophyllum, so characteristic of the Palz- 
OzoOic period, is to; be pred : 
and amongst the Zadulata 
we have similarly the first appearance, in force at any rate, 
of the widely-spread genus Savosites — 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. 
