Crustacea. ] LOWER PALASOZOIC ARTICULATA. 173 
and puncta, and the extremities of the pleuree with elevated striz parallel to their anterior margin. Average 
length of head one and a half inches. 
The width (about one and a half lines) and flatness of the segments, and the narrow, inflated and much- 
inflected front, distinguish this species. 
Position and Locality. — Abundant in the Coniston limestone at Coniston Water-Head, Lancashire ; 
Coniston limestone of Sunny Brow, near Coniston; Blain y Cwm, W. of Nantyre, Glyn Ceiriog. 
Explanation of Figures.—P\. T. G. fig. 33. Part of cephalic shield, natural size, from Coniston. Fig. 34 
thorax, and fig. 35 pygidium, same locality. 
DysPLANuUS CENTROTUS (Dal. Sp.) PI. 1.E. fig. 19. 
Ref. and Syn.—IMlenus centrotus Dal. and Portlock. Geol. Rep. Pl. x. f. 3 to 6. 
= Illenus Bowmani (Salt.) Mem. Geol. Sury. Vol. II. t. 8. fig. 1, 2. 
Sp. Ch.—Thorax little more than half the length of the head, of nine short weak segments, axal 
portion about equal to the pleurze, which are abruptly bent downwards and backwards at rather less than 
half their width from the axis; cephalic shield slightly longer than the pygidium, about one-third wider 
than long, moderately convex, axal furrows one-third the length of the head, slightly arched, the enclosed space 
one-third wider than the cheeks ; eyes small, their own length in advance of the posterior margin of the head, 
and nearer to the external margin than to the glabella; pygidiwm one-sixth wider than long, gently convex 
at the anterior portion, becoming more depressed towards the margin, axal furrows barely forming a short 
rounded sinus on each side; average length of head one inch three lines. 
I see no grounds for Mr Salter’s opinion (Mem. Geol. Sury.), that the present fossil had not the lateral 
angles of the cephalic shield prolonged, and consequently differed from the J. centrotus (Dal.) All the Eng- 
lish and Irish specimens recorded, or which I have seen, have this part broken; and when we consider that in 
all other respects the species are identical, I think Col Portlock is correct in uniting them; the more so, 
as in the foreign perfect examples rounded angles to the buckler are only found in those species having ten 
joints to the thorax, while the JZ. centrotus forms the type of the genus Dysplanus from having but nine segments, 
like the example before us, and the cephalic angles prolonged. The very circumstance of the angles being 
broken off seems rather to indicate their having been prolonged, the projecting portion acting as a lever, while 
if they were xatwrally rounded they would probably present the rounded angle entire, like that of the pygidium. 
The greater length of the pygidium in proportion to its width distinguishes it from that of the J. crassi- 
cauda. 
Position and Locality —vVery common in the limestone of Llanwddyn in the Berwyn mountains ; rare, 
of large size in the Coniston limestone of Llandeilo, Caermarthenshire. 
Explanation of Figures.—P1.1. EK. fig. 19, Entire specimen, natural size.—Fig. 19 a. Head, natural size, 
of a rolled-up specimen. 
Genus. FORBESIA (A/'°Coy). 
Ref.—M Coy, Synop. Sil. Foss. Irel. 
Gen. Char.—Head semielliptical, with distinctly defined glabella ; eyes finely reticulated ; eye-Jines nearly 
parallel, cutting the middle of the posterior margin; body-rings ten, facets large, pleural furrow slightly 
oblique, not reaching the margin; pygidium smaller than the head, with duplicate lateral furrows, and a 
distinct conical articulate axis. 
Two Subgenera: Ist, Morbesia; 2nd, Proetus. 
Subgenus. FORBESIA (M*Coy). 
Ref. and Syn.—Monia (Burmeister). Ray 2nd Ed. Organ. of Trilobites. 
Gen. Char.— Cephalic shield semielliptical, the lateral angles produced backwards into long spines; glabella 
oblong, contracted in the middle, with three small transverse segmental furrows on each side, the basal 
