Belt - On the Fauna of the " Lingula Flags'^ 7 



The Lower Cambrian rocks, with their most abundant fauna/ are 

 followed by strata, containing but two genera of trilobites — Aqnosius 

 and Olenns. Of these, Agnostus comes from below and passes on 

 upwards unchanged in type. Olenus, on the contrary, only remains 

 true to its typical form through the Maentwrog beds, in which 

 it has three British representatives. In the Upper Festiniog and 

 Lower Dolgelly beds it branches off in two directions. In one, 

 through Conocoryphef bucephala towards the pseudo- Conocoryphe and 

 Dihelocephali of the Upper Dolgelly beds ; in the other, through 

 ParaboUna spinidosa towards the Pelturce and SpJioerojjhtJialmi of the 

 same strata. This relation is shown in the following diagram : — 



I — JDikelocephalm. 

 — Conocoryphe ? — 

 I ' — Conocoryphe ? 



Olenus — 



I I — Sphocrophthahnus. 



— Farabolina — 



' — Teltma. 



Many links in the chain are wanting, but this much is certain, 

 that the typical Oleni of the lower strata are followed by two genera ; 

 one of which — Conocoryphe ?, with entire pygidium and falcate 

 facetted pleurae, is intermediate between the Oleni below and the 

 Conocoryphe ? and Dihelocephali above ; the other, ParaboUna with 

 serrated pygidium and spinous, unfacetted pleurae, intermediate 

 between the Oleni below and the Sphcerophthalmi and Peltura above. 

 This result is obtained, not by picking out from a number of species 

 those that could be brought within such a generalization, but by 

 using all the species, and placing them in the order, as to time, in 

 which they occur in the strata. 



Some interesting relations between the fauna of the Maentwrog, 

 Festiniog and Dolgelly beds, and that of the underlying Lower 

 Cambrian rocks on the one hand, and of the overlying Tremadoc 

 and Lower Silurian strata on the other, still remain to be pointed 

 out. Between the deposition of the Upper Menevian and the Lower 

 Maentwrog beds, the abundant fauna of the Lower Cambrian period 

 disappears, and our next horizon shows only a few species of 

 Agnostus and Olenus ; the latter, pigmy representatives of the giant 

 Paradoxides of the preceding age. All on through the Maentwrog, 

 Festiniog, and Dolgelly epochs, the various Trilobites that succes- 

 sively appear either belong to or are closely allied to these two 

 genera. The fauna is a compact and homogeneous one. We have no 

 intrusions of new types of structure, but the more recent forms are, 

 if I may use the phrase, the natural evolution of the older. When 

 we pass on upwards into the Tremadoc and Arenig epochs, a great 

 and apparently sudden change takes place. 



Large Trilobites belonging to the AsaphidcB now first appear, and 

 then the Cheiruridce, Trinucleidce, and Calymenidce come upon the 

 stage. Of these there has been no development in the area under 



1 See ante, Part I., "Table of the Cambrian Rocks showing the range of the 

 Genera," Vol. IV. p. 495. 



