UPPER CAMBRIAN.—TREMADOC SLATES 
483 
slates were first examined by Sedgwick in 1831, and were 
re-examined by him and described in 1846,* after some fos¬ 
sils had been found in the underlying Lingula flags by Mr. 
Davis. The inferiority in position of these Lingula flags to 
the Tremadoc beds was at the same time established. The 
overlying Tremadoc beds were traced by their pisolitic ore 
from Tremadoc to Dolgelly. No fossils proper to the Tre¬ 
madoc slates were then observed, but subsequently, thirty- 
six species of all classes have been found in them, thanks to 
the researches of Messrs. Salter, Homfray, and Ash. We 
have already seen that in the Arenig or Stiper-Stones group, 
where the species are distinct, the genera agree with Silurian 
types; but in these Tremadoc slates, where the species are 
also peculiar, there is about an equal admixture of Silurian 
types with those which Barrande has termed primordial.” 
Here, therefore, it may truly be said that we are entering 
upon a new domain of life in our retrospective survey of the 
past. The trilobites of new species, but of Lower Silurian 
genera, belong to Ogygia^ Asaphiis^^eiXi^ Cheiriiriis; whereas 
those belonging to primordial types, or Barrande’s first fau¬ 
na as well as to the Lingula flags of Wales, comprise Dike- 
locephalas^ Coiiocoryphe (for genera see Figs. 577 and 581),f 
Olenus^ and Angelina, In the Tremadoc slates are found 
Bellerophon^ Orthoceras^ and Cyrtoceras^ all spe¬ 
cifically distinct from Lower Silurian fossils of 
the same genera: the Pteropods Theca (Fig. 
568) and Conularia range throughout these 
slates; there are no Graptolites. The Lingula 
{Lingulella) Lavisii ranges from the top to 
the bottom of the formation, and links it with 
the zone next to be described. The Tremadoc 
slates are very local, and seem to be confined 
to a small part of North Wales; and Prof. Ram¬ 
say supposes them to lie unconformably on the 
Lingula flags, and that a long interval of time er Tremadoc* 
elapsed between these formations. Cephalo- Tremadoc. 
poda have not yet been found lower than this group, but 
it will be observed that they occur here associated with 
genera of Trilobites considered by Barrande as characteris¬ 
tically Primordial, some of which belong to all the divisions 
of the British Cambrian about to be mentioned. This ren¬ 
ders the absence of cephalopoda of less importance as bear¬ 
ing on the theory of development. 
* Quart. Geol. Journ., vol. iii., p. 156. 
t This genus has been substituted for Barrande’s Conocephalus, as the latter 
term had been preoccupied by the entomologists. 
Fig. 568. 
