12 EXPLANATION OF THE PLATES. 
British Trilobites, Pal. Soc., p. 164, pl. xxiv., figs. 13, 14. With respect to 
this fossil Mr. Salter remarks, that ‘it is not certain that this is identical 
with Professor M‘Coy’s very imperfect specimen ; but I wish to keep that 
species in mind, as it may eventually be included with 4. Homfray.” 
Position and Locality.—_Urrer Tremapoc. Pen-y-Clogwyn, south of Port- 
madog, in flinty slate, much compressed by cleavage, Garth, Penrhyn ; near 
Llanerch, by the roadside towards Treflys; Tyddyn-dicwm above Pen- 
morfa. 
Fig. 4.—_NiospE Homrrayl, Salter. 
British Trilobites, by J. W. Salter, Esq., Pal. Soc., 1865, p. 143, pl. xxii. 
figs. 3-12; Memoirs Geological Survey, vol. iii., pl. vi., fig. 5. 
Reduced one-third from the figure in Memoirs Geological Survey, vol. iii., 
Levies oD: 
towne TrEMADOoc SLats.—Penmorfa Church, Tremadoc; Castle Dend- 
raeth, near Maentwrog ; Ogof-ddu ; Borthwood ; Tyn-y-llan. 
Passage beds.—Ulanerch ; North Wales. Memoirs Geological Survey, vol, iii., 
pp. 253, 316. 
Fie. 5. PsiLocePHALus INNOTATUS, Salter. 
Memoirs Geological Survey, vol. iii., p. 315, pl. vi., figs. 9-12; Brit. Trilob. 
Pal. Soc., 1866; p. 175, pl. xx., figs. 13-19. . 
This small Trilobite rarely more than an inch, although sometimes an inch and 
a half long, described by Mr. Salter as the most abundant fossil in the Lower 
Tremadoc beds, in company with Niobe Homfrayi, a much less common 
species. ‘The form is very convex, especially in the head, which is but 
slightly lobed, the tail more strongly so ; and the body, or thoraz, is strongly 
trilobed throughout. 
Lower Tremapoc.—Penmorfa, Tremadoc ; and the neighbourhood of Borth 
and Moel-y-gest; e. g. Borthwood, Tyn-y-llan, Tyddyn-llwyd Farm, &e., 
North Wales. 
Fig. 6.—CHEIRuRuUS FREDERICI, Salter. 
Memoirs Geological Survey, vol. iii., p. 322, pl. viii., fig. 1; Brit. Trilob., 
Pal. Soc., 1866; p. 74, pl. v., figs. 18-21. 
This Trilobite, Mr. Salter remarks, only found as yet in the upper part of 
the Tremadoc Slates, is a member of a genus more characteristic of higher 
beds of the Lower Silurian. It is occasionally as much as three and a 
half inches long, rather wide and much compressed. 
MOLLUSCA.—BRACHIOPODA. 
Fie. 7.—a, b. Lineua LEpPIs, Salter, sp. 
Lingulella lepis, Salter, Memoirs Geological Survey, vol. i., p. 354, wood- 
eut, fig. 11. Lingula lepis, Davidson, Silurian Brachiopoda, Pal. Soc., 
1866, p. 54, pl. iil., figs. 55-9. 
Our figures are taken from the two upper ones in the woodcut, Memoirs 
Geological Survey, p. 334, fig. 11. Mr. Salter describes it as a smaller and 
rounder form than L. Davisu, the beak being pointed, and the surface 
