652 -STRATIGRAPHICAL GEOLOGY. [Boox VI. 
which appear among these ancient rocks, have persisted with but little | 
change, at least in external form, through the whole of geological 
time and are alive still. Other genera are Lingulella (Fig. 321), 
Obolella (Fig. 320), Kutorgina, and Orthis (Fig. 321). Every class of 
the true mollusca had its representatives in the Cambrian seas. The 
lamellibranchs occurred in the genera Ctenodonta (Fig. 321), 
Palzarea, Davidia, and Modiolopsis. The gasteropods were present 
in the heteropod genus so characteristic of Palzozoic time, Bellero- 
phon (Fig. 321). The pteropods were represented by the genera 
Theca (Fig. 3820) and Conularia, the cephalopods by Orthoceras 
(Fig. 321). 
Taking paleontological characters as a guide in classification it 
has been proposed to group the Cambrian system according to the 
distribution of characteristic trilobites, into two divisions—the lower 
(Harlech or Longmynd and Menevian rocks of Britain) termed 
Paradoxidian, and the upper (Lingula and Tremadoc) Olenidian. 
§ 2. Local Development. 
Britain.'—The area in which the fullest development of the oldest 
known Paleozoic rocks has yet been found is undoubtedly the principality 
of Wales. The rocks are there much thicker than in any other known 
region, they have yielded a more abundant fauna, and they possess 
additional importance from the fact that they were the first strata of such 
- antiquity to be worked out stratigraphically and paleontologically. As 
already stated, they were named Cambrian by Sedgwick, from their 
extensive development in Cambria or North Wales, where he originally 
studiedthem. ‘Their true base is nowhere seen. According to Sedgwick 
and subsequent observers (especially Messrs. Hicks, Hughes, and Bonney) 
they rest unconformably upon a set of igneous and metamorphic Archean 
rocks, so that their base at any given locality must be merely a local 
phenomenon. Professor Hughes believes that a strong conglomerate and 
erit generally mark the base of the Cambrian series.? According to 
Professor Ramsay on the other hand, the base of the Cambrian series is 
either concealed by overlying formations or by the metamorphism which 
he thinks has converted portions of the Cambrian series into various 
crystalline rocks. Starting from the lowest observable horizon among 
these ancient sedimentary deposits, the geologist can trace an upward 
succession through many thousands of feet of grits and slates into the 
Silurian system. Considerable diversity of opinion has existed, and 
still continues, as to the line where the upper limit of the Cambrian 
system should be drawn. Murchison contended that this line should be 
placed below strata where a trilobitic and brachiopodous fauna begins, 
and that these strata cannot be separated from the overlying Silurian 
" See Sedgwick’s Memoirs in Quart. Journ. Geol. Soc., vols. i. ii. iv. viii., and his 
«Synopsis of the Classification of the British Paleozoic Rocks,’ 4to0, 1855; Murchison’s 
«“Siluria,” and Ramsay’s “ North Wales,’ in Geological Survey Memoirs, vol. iii., and 
papers by Salter, Harkness, Hicks, Hughes, and others in the Quart. Journ. Geol. Soc. 
and Geol. Mag., to some of which reference is made below, 
2 Q. J. Geol. Soc. xxxiv. p, 144. 
