xx DESCRIPTIVE REMARKS. 
fig. 4, remarkable for its globular head and large eyes, also by having 
two nodular prominences on the third segment, and a triangular tail, is 
abundant in the black slates of the Lower Llandeilo, west of the Stiper 
stones, Shropshire. 
The most easily recognised, as well as perhaps the most charac- 
teristic genus of Trilobites in this Formation, is that of Trinucleus, 
moderately small in size, but important in point of numbers, as some 
of the species are; the expanded border to the head, usually orna- 
mented with radiating lines or perforations, its stunted form and long 
cheek spines readily distinguish it. We have included two species on 
Plate vi. Zrinucleus fimbriatus, fig. 5, originally figured in the ‘ Silurian 
System’ (head only), pl. xxiii., fig. 2 (as Mr. Salter informs us), is not 
the same Fossil as that figured so long ago as 1698, by Dr. Llhwyd in 
the ‘‘ Philosophical Transactions” as Trinucleum fimbriatum, that being 
the common Llandeilo speeies, Z. concentricus or T. Caractact.* The 
fossil from which our figure was taken was collected at Builth, in Rad- 
norshire, where it is very plentiful. TZrinucleus Lloydit, fig. 6, is a 
species also originally figured in the Silurian System, pl. xxiil., fig. 4, 
and since by the Geological Survey in Decade vii., pl. vii. The figure in 
the Silurian system, as remarked by Mr. Salter, ‘‘ although characte- 
ristic, was taken from a specimen having but five body, or thoracic 
rings;”’ he supposes the deficiency due to accidental growth. It is 
said to be confined to Caermarthenshire and the mining district of Shrop- 
shire, where itisfrequent. Belonging to the same small group is Ampyx 
nudus, a species remarkable for its extremely developed cheek spines and 
pointed frontalspine. Itaffords a good illustration of the caution necessary 
in naming and describing, having been originally figured in the ‘ Silurian 
System’ as a species of Trinucleus, in consequence of its imperfect 
character, from being deprived of its spines, and under the supposition 
that it had lost the expanded margin peculiar to that genus; speci- 
mens in a more perfect condition were afterwards collected at Builth, 
where it is not uncommon, proving it to belong to the genus Ampyx; 
and it was again described by Professor E. Forbes, then Paleontologist 
to the Geological Survey, in Decade ii., Plate 10, the drawings to illus- 
trate it being executed by the Author, who has repeated the principal 
figure on Plate vii. fig. 7. No facial suture has been observed in this 
genus, and it is said to be without eyes. 
The remaining Trilobite, Calymene duplicata, belongs to a genus 
which ranges through the whole of the Silurian strata from the Lower 
Llandeilo to the Ludlow, and is, according to Mr. Salter, one of the 
most typical forms of Trilobite, and not known beyond the British Isles. 
It is abundant at Builth, and occurs also in South Wales and Shropshire. 
Various classes of the Mollusca, some of which appear for the first 
time in this Formation, are represented on Plate viii., most numerous 
amongst them being the shells of Brachiopoda, which includes several 
species of Orthis; ofthese Orthis alata, fig. 4, is confined to the Llandeilo 
* Memoirs of the Geological Survey, Decade vii,, Explanation to Plate vii., p. 5. 
