14 Proceedings. 
representatives of a deep-sea fauna which must have been pre- 
ceded by another richer fauna of which we have no knowledge. 
(2nd.) In the Ordovician Fauna trilobites continue to abound. 
Brachiopods are very abundant and preponderate over the true 
molluscs. Graptolites are very numerous and exhibit a great 
variety of forms. 
The trilobites of the 2nd Fauna are much more compact than 
those of the 1st; the pleura are more closely appressed. The 
head, body, and tail are nearly equal in size. The eyes large, 
well developed, and faceted. 
(8rd.) In the Silurian Fauna trilobites no longer prepon- 
derate and the forms that occur are specialized. Echinoids, 
molluscs, and brachiopods are abundant. The graptolites con- 
sist of but few genera, but the species are numerous. At the 
end of the series fish appear. 
With the risk of going outside my subject, I must complete 
these introductory remarks by referring to the subdivisions of 
the Ist and 2nd Faunas. They are described by their cha- 
racteristic trilobites. 
CAMBRIAN. 
Fauna. Formation. 
Olenellus fauna. Harlech and Caerfai. 
Paradoxides ,, Solva and Menevian. 
Olenus i Lingula Flags. 
Ceratopyge ,, Tremadoc Slates. 
ORDOVICIAN, 
Phyllograptus fauna. Arenig. 
Placoparia ‘5 Llanvirn. 
Asaphus ss Llandeilo. 
Trinucleus - Bala. 
We will now consider in detail the lithological character and 
the paleontological contents of the Tremadoc Slates of the 
four chief districts where they are exposed. 
We find these beds in North Wales, near Tremadoe and 
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