8 EXPLANATION OF THE PLATES. 
PLATE IV. 
FOSSILS OF THE LINGULA BEDS OR FLAGS. 
CRUSTACEA.—PHYLLOPODA. 
Fra. 1.—a, b. HyMENOCARIS VERMICAUDA, Salter. 
Proceedings of the British Association, 1852, p. 58; Memoirs Geological 
Survey, vol. iil., p. 293, pl. i, &. 
a. From Memoirs Geological Survey, vol. iii., pl. i., fig. 2, representing 
several of these remarkable Crustacea, somewhat altered by pressure. 
b. Restored outline, from the same plate, fig. 4, reduced in size, showing the 
large carapace ; abdomen, consisting of nine transverse segments, the last 
being furnished with three pairs of unequal lanceolate appendages, or tail 
spines. 
P With respect to its occurrence, Mr. Salter remarks :—‘‘ Hymenocaris 
appears to be a far more plentiful fossil than at first supposed ; it evidently 
affected sandy shallows, rather than a deeper and muddier sea bottom.” 
Memoirs Geological Survey, vol. iii., p. 248. 
Linauia Fiags.—Abundant in the upper portion of the true Lingula Flags. 
Lower Lineuta Fiacs.—In fine grey sandstones, with Lingulella Davisii ; 
Gwern-y-bareud, Moel-hafod-owen, and other places near Dolgelly; at 
Tremadoc, Ffestiniog ; and at Pont Seiont, Caernarvon, fragments, appa- 
rently of this species, occur.—Memoirs Geological Survey, vol. iii., pp. 
293-4. Other localities mentioned in list of Lower Lingula Flag fos- 
sils :—Pentrefelen, west of Penmorfa ; Borth, west of Portmadoc; Jbid., 
p. 247. 
CRUSTACEA.—TRIL OBITA. 
Fig. 2.—a, b. AGNOSTUS PRINCEPS, Salter. 
A. pisiformis, Salter, in Siluria, third edition, pp. 45, 53; Foss, 4, f. 4 ; 
Do., 9, f. 9; Decade Geological Survey, No. 11, pl. i, figs. 1-5; 
Memoirs Geological Survey, vol. ili., p. 296, pl. iv., figs, 2-11, pl. v. 
fig. 1. 
a. Origa In highly cleaved and crumpled slate, the fossils being cu- 
riously distorted and elongated in opposite directions. From the collec- 
tion of the Geological Survey of Ireland; Lower Lingula Flags, Maen- 
twrog, near Ffestiniog, North Wales. 
b. Enlarged view of an entire specimen of these small and remarkable Trilo- 
bites, in which the cephalic (head) and caudal (tail) shields are nearly 
equal ; the body rings are two only, and they have no eyes or facial suture. 
These fossils are, as Mr. Salter remarks, the lowest and most rudi- 
mentary form of Trilobite, and amongst the oldest of known fossils. ‘* Me- 
moirs Geological Survey,” vol. ili., p. 296. 
