18 
MIDDLE CAMBRIAN. 









cae ae Reference to McCoy’s ¥ 
ola fc Synofels® and Figures offGenee Names and References; Observations, &c. Numbers and Localities, 
G Theca (Centrotheca) cuspidata, Salter (Mem. | b. 336, Portmadoc, (Mr. 
case Geol. Sury. Vol. m1. pl. 10, fig. 25). A long Homfray.) 
horn projects on each side from the mouth. 
& Theca trilineata, MSS, A small species with | 47 ne ne 
longitudinal lines. Hoiniten 
y:) 
G Conularia Homfrayi, Salter (Mem. Geol. Surv. | Garth, Portmadoc, (Mr. 
Gi Vol. m1. pl. 10, figs. 11—13). A very large | Ash.) b. 323, 824, 325 
species, so slightly calcareous as to appear | figured specimens in above 
mere membrane on the slate. The pucker- | work. (Mr. Homfray.) 
ing of the surface is probably due to unequal 
contraction. See Lower Ludlow Rocks. 
& Bellerophon arfonensis, Salter (1. c. p. 349, pl. | Garth, Portmadoc. b. 328, 
10, figs. 6—8). Squamose lines of growth, | 341 (Mr. Homfray, figured 
remote and rather regular. specimens). 
Gi Bellerophon multistriatus, Salter (1. c. p.350, pl. | Garth, Portmadoc, (Mr. 
10, figs. 9, 10). With close decussating strie. | Homfray.) 
CEPHALOPODA. 
G Orthoceras sericeum, Salter (1. c. Vol. 111. p.356, | Garth, Portmadoc. (Pre- 
ne pl. 10, figs. 4, 5). This, and the Cyrtoceras | sented by Mr. Salter.) 
Gi precox of the same work, are the oldest | b. 321, 322, figured speci- 
known forms of the Nautiloid, or shelly | mens in the work quoted. 
| Cephalopod group. Cephalopoda became | (Mr. Homfray.) 
abundant in the succeeding period—the Bala 
| group, even in its lower portion: and a few 
are known in the Arenig Group. b. 321 
shews the septa. 
The great break, in organic life, between the ‘Tremadoc Slate’ and the ‘Arenig or Skiddaw Group,’ has 
disposed me, ever since I worked out their respective faune in the Tremadoe district in 1853, to regard the 
next overlying, or ‘Arenig Group,’ as the base of the great Upper Cambrian Group of Prof. Sedgwick. 
Sir R. I. Murchison, in endeavouring to bring it first within the Ffestiniog or Lingula Flag Group, and later, 
among the Llandeilo or Lower Bala Group, has involved greatly the fossil evidence. 
But the Arenig or Skiddaw 
Group (Lower Llandeilo of Murchison) is peculiar, with the facies of the Lower Bala or Llandeilo Group, yet 
with wholly distinct species. 
To keep this Catalogue in harmony with the Synopsis, published in 1851—3, it is 
placed here as Prof. Phillips also regards it, as the terminal member of the Middle Cambrian. 
The fossil evidence would permit us, with Lyell, to commence the Upper Cambrian (or Lower Silurian of 
Murchison) with this Group, which is well represented in the Stiper Stones district; though the fossils of that 
district were not described till 1859 by Murchison and myself, long after the Arenig Group, with its few fossils, 
found by Prof. Sedgwick in 1848, was established. The group was further illustrated by the fossils found in 
Skiddaw by Prof. Sedgwick, and described by McCoy, previous to 1851 (though the relative age of the rock 
was not then fully known). The right of nomenclature rests therefore with the Woodwardian Professor. 
As the majority of the fossils, in both these transitional groups (Tremadoc and Arenig), have been first 
described by me, I may say that the Tremadoc Group seems to be the natural termination of the Ffestiniog or 
Middle Cambrian series; and the Arenig Group the true base of the Upper Cambrian (or Lower-Silurian 
of Sir R. I. Murchison). It is here treated as an intermediate Group. (J. W. S.) 
