DESCRIPTIVE REMARKS. XvVii 
have figured on Plate iv., three species—O. micrurus, fig. 4; O. cata- 
ractes, fig. 5; and O. humilis, fig. 6; the two former species both oc- 
curring in the black shales of the Lower Lingula Flags, North Wales; ° 
and the latter, a small species first described by Professor Phillips, com- 
mon in the black shales of the Upper Lingula Flags, Malvern. The 
pygidium or tail of a Trilobite called Dikelocephalus (?) Celticus, fig. 7, 
and portions of the head and tail of another, Conocoryphe invita, fig. 8, 
both from Upper Lingula Flags, North Wales, complete the illustra- 
tions of the Crustacea given on Plate iv., which form an important and 
peculiar group in the strata of this Formation. 
Fossits oF THE TREMADOC SLATE. 
These strata, as we are informed by Professor Ramsay,* were first 
described by the Rey. Professor Sedgwick, on special lithological grounds, 
as long agoas 1846; Mr. Salter having since, from the study of their 
Fossils, considered them entitled to rank as a distinct formation—a 
claim now acknowledged by the Geological Survey. They are described 
as a very local formation, immediately succeeding the black Lingula 
slates, and as being at present only recognised in certain parts of 
Merionethshire and Caernarvonshire, North Wales ; and in Pembroke- 
shire, South Wales. The general range and character of these beds 
- are shown by Professor Ramsay in his ‘‘ Memoir on the Geology of 
North Wales,”’ before referred to, and the'Fossils, separated into two 
zones, described by Mr. Salter, and figured in the Appendix to the same 
volume; a selection from these has furnished us with the materials for 
Plate v., Trilobites being the most abundant; of these sixteen different 
kinds are enumerated in the list of species. The most characteristic, 
according to Mr. Salter, are Niobe Homfrayi, fig. 4; and Psilocephalus 
innotatus, fig. 5 in the lower division; and Angelina Sedgwick, fig. 2, 
and Asaphus affinis (or A. Homfray?), in the upper. The remaining 
Fossils are almost entirely Mollusca—Lingulella Daviswi (Plate iii., fig. 
8), and a smaller shell, Lengula lepis (pl. v., fig. 7), also called Lingu- 
lella by Mr. Salter. Mr. Davidson, however, considers it to possess 
characters so similar to that of Lingula proper, that he places it pro- 
visionally with the last-named genus.{ Both these species are noticed 
as occurring in the Linguia Flags, and are stated by Mr. Salter to be 
the only Fossils which ascend from that lower horizon. The other shells 
are Bellerophon Arfonensis, fig. 8; Theca operculata, fig. 9; and Conu- 
laria Homfrayi, all frequent Fossils in these beds, the genus Theca 
being very predominant; Orthoceras and Cyrtoceras are also said to 
occur, appearing for the first time in these deposits in Britain. 
* Mem. of the Geol. Survey, vol. iii., p. 7. 
+ Ibid., p. 253. 
t British Silurian Brachiopoda, p. 54. 
Cc 
