
Pant IL. Sect. ii. §2] SILURIAN. 667 
has been found to hold good over a large part of the world. The sub- 
joined table shows the arrangement and nomenclature of the various 
subdivisions of the Silurian system : 
Feet. 
7. Ludlow group : : : . 51950 
Upper Silurian. ;6. Wenlock group : ; 4 . 1600 
5. Upper Llandovery group . - . 1500 
4, Lower Llandovery group . : . 1900 
Marg ee 3. Bala and Caradoc group . - . 6060 
Lower Silurian. 2. Llandeilo group . : ‘ » 2500 
1. Arenig or Stiper Stone grou : . 4000 
——— 
Approximate average thickness = 18,550 
Lower Silurian.—1. Arenig or Stiper Stone Group.—These rocks 
consist of dark slates, shales, flags, and bands of sandstone. They are abun- 
dantly developed in the Arenig mountain, where, as originally described 
by Sedgwick, they contain masses of associated porphyry. Throughout 
that district they have been deposited at a time when streams of lava and 
showers of volcanic ashes were thrown out in great quantity from sub- 
marine vents. They contain an abundant suite of organic remains (63 
genera and 150 species), of which only eleven genera and sixteen species 
are common to the Tremadoc beds below, while eight genera and nine 
Species pass up into the next group. New genera of trilobites make their 
appearance in these rocks (Aiglina, Barrandea, Calymene, Homalonotus, 
Mlznopsis, Illznus, Phacops, Placoparia, Trinucleus). Eight species of 
pteropods (Conularia, Theca), eighteen species of brachiopods (Lingula, 
Lingulella, Obolella, Discina, Siphonotreta, Orthis), six lamellibranchs, four 
gasteropods, and five cephalopods have been found; but the most abun- 
dant organisms are the graptolites, of which the Arenig rocks of St. 

1, Orthoceras cereesiense (Hicks); 2, Bellerophon Ilanvirnensis (Hicks); 3,-Orthis 
ao (Dalm.) (enlarged); 4, Redonia anglica (Salt.); 5, Paleearca amygdalus 
alt.). 
David's, in Pembrokeshire, have yielded forty-two species, which belong 
to eighteen genera, including Didymograptus, Tetragraptus, Diplograptus, 
Dendrograptus, and Callograpius.1 This sudden and great development 
1 Hicks, Quart. Journ. Geol. Soc. xxxi. 167, Hopkinson and Lapworth, ¢bzd. p. 635 
Etheridge, zbid. xxxvii. p. 89, 
