THE SILURIAN SYSTEM. 



671 



Vale of the Towy. 



carbonate of lime passing occasionally into an impure limestone. The flags are seen in 

 broken and detached masses on the right bank of the river Towy from Llandeilo to 

 Caermarthen, a distance of fifteen miles ; often in a highly inclined, vertical, and 

 contorted position, and reversing their direction repeatedly in a very short space, a me- 

 morial of the powerful convulsions with which the strata have been agitated. These 

 commotions of the past have contributed to endow the surface with great beauty, throw- 

 ing it into knolls, which, in the vale of the Towy, are richly covered with woods, in some 

 instances apparently going back to the time of the Silures. The annexed sketch of the 

 scenery of this valley represents part of the domestic domain of Dinas-fawr, the residence 

 of the ancient princes of South Wales, now the property of Lord Dynevor. 



The most common or remarkable fossils of the 

 Llandeilo flags, besides a few shells, are the mol- 

 lusca (Fig. 2.), which has a diameter of nearly 

 two inches, and occurs near Llandovery in black 

 schistose beds of passage from the Silurian into the 

 Cambrian rocks ; and the chain-coral catenipora 

 escharoides, a species which sometimes forms hemi- 

 spherical masses more than a foot in diameter. 



But the characteristic fossils are the trilobites 

 (three-lobed), a name referring to their structure, 

 consisting of an oblong body, divided transversely 

 into three principal parts, and longitudinally into 

 three lobes. These fossil bodies have long been 

 known under the provincial title of Dudley insects, or locusts, a district where they abound ; 

 and the characteristic terms of species display the perplexity of naturalists respecting them. 

 Asaphus, from a<ra0j)e, obscure; Calymene, from KeKaXvppevr] 9 concealed; Paradoxus, from 

 7rapuooc, wonderful ; and Agnostus, from ayi/woroe, unknown. In no strata more recent 

 than the carboniferous has any trilobite yet been discovered ; but a few genera and species 

 occur in that system. The Silurian rocks have been styled the " great trilobitic series," 

 the "grand mausoleum of these ancient beings," from their prevalence, some of which 



2. Lituitis Cornuarietus. 



