204 
SILUEIA. 
[Chap. IX. 
or is more widely diffused in geographical space, than the long-known Dudley 
fossil, Calymene Blumenbachii, Brongn. When this Trilobite was described 
in the e Silurian System,' it was considered to be typical of the upper rocks 
only, whether of Wenlock or of Ludlow age ; but it has since been found in 
abundance in the lower strata of the Caradoc formation, even near Snowdon, 
and I have myself procured it from beds of Caradoc Sandstone in Shropshire. 
Again, a variety of this species occurs in the Lower Silurian rocks of Ty- 
rone; and thus, like many examples of the other classes of marine animals 
which lived in primeval days, this Crustacean links together the Lower and 
Upper Silurian in one system of life. (See PI. XVIII. f. 10. The head is re- 
presented, Foss. 13. f. 1.) It has also been found in Sweden, Bohemia, North 
America, and Eastern Australia (?), and always in company with similar gene- 
ric forms. Acidaspis Brightii, PI. XVIII. f. 7, and Cheirurus bimucronatus, 
PI. XIX. f. 11, also occur in both divisions, — the former, however, but rarely, 
the latter abundantly. So do also Cyphaspis megalops, M'Coy, and Stauroce- 
phalus Murchisoni, Barr., a small Trilobite with a globular head and serrate tail. 
The Lower Silurian fossil, PI. II. f. 3, 4, called Asaphus Vulcani in the ' Silu- 
rian System,' is a species of Homalonotus, a genus formerly thought to be 
peculiar to the Upper Silurian, but of which several examples are now known 
in the lower division. 
Fossils (46). Trilobites, &c, typical op Lower Silurian Eocks. 
1. Asaphus Pow- 
isii, Murch. 2. 
Illasnus Davisii, 
Salter. 3. Trinu- 
cleus concentricus, 
Eaton(Trin. Carac- 
taci, Sil. Syst.). 4. 
Calymene brevi- 
capitata, Portlock. 
5. Lichas laxatus, 
M'Coy. 6.Agnostus 
trinodus, Salter. 7. 
Beyrichia compli- 
cata, Salter, en- 
larged. (The last is 
not a Trilobite, but 
a Bivalved Phyllo- 
podousCrustacean.) 
The principal forms which have proved unerring indices of the Lower Silurian 
of Britain are Paradoxides, Olenus, Agnostus, Conocoryphe, Dikelocephalus, 
and the true Asaphus (such as A. Powisii, Foss. 46. f . 1 ; A. tyrannus, PI. I. 
f. 4, 5, and PI. II. f. 1), with five species of Trinucleus, PI. IV., all of which 
were formerly published as true Lower Silurian types. 
Trinucleus concentricus (Caractaci, Sil. Syst.), PI. IV. f. 2-5, Foss. 46. f. 3, and 
Foss. 47, has a very extensive Lower Silurian range both in Europe and America. 
This species varies greatly in form and markings ; and, being a very common 
species, it is met with in every variety of distortion and compression, so that 
it might often at first sight be referred to a number of different species. 
