110 STRATIGKAPHICAL GEOLOGY 



Gradually, however, it has been found that the fauna of the 

 Llanvirn Beds has more than a local value and is really distinct 

 from those above and below. A portion of the Llanvirnian outcrop 

 was mapped by Messrs. Marr and Roberts in 1885, 3 and other 

 portions subsequently by the Geological Survey, with the result 

 that in a recent memoir Dr. Strahan announces that the only 

 possible course is to restrict both the Arenig and Llandilo Series, 

 and to establish an intermediate Llanvirn Series. 4 



Dr. Marr has also advocated the separation of the Upper Bala 

 from the rest of that series because it contains many species which 

 do not occur or are rare in the lower beds, but the true value of 

 this Ashgillian fauna has yet to be proved, and in this volume I 

 shall not recognise more than four main divisions in the Ordovician 

 System, namely : 



4. Bala Series (Ashgillian and Caradocian). 



3. Llandilo Series or Llandeilian. 



2. Llanvirn Series or Llanvirnian. 



1. Arenig Series or Arenician. 



B. LIFE OP THE PERIOD 



Taken as a whole, the special features of the Ordovician fauna 

 are (1) the abundance of branched and biserial graptolites belonging 

 to the families Dichograptidce, Dicranograptidce, Glossograptidce, 

 Leptograptidce, and Diplograptidce. Of these the first includes the 

 following genera Dichograptus, Tetragraptus, Loganograptus, Didy- 

 mograptus, and Phyllograptus ; in the Dicranograptidae are Dicello- 

 graptus and Dicranograptus. The Glossograptidse include Glosso- 

 graptus and Lasiograptus. The Leptograptidse include Leptograptus 

 and Nemagraptus ( = Ctznograptus), while Diplograptus, Crytograptus, 

 and Climacograptus belong to the Diplograptidae. 



(2) The appearance of many new trilobite genera *sEglina, 

 Acidaspis, Ampyx, *Amphion, Asaphus, Calymene, Encrinurus, 

 Harpes, Homalonotus, Illcenus, Lichas, Phacops, *Placoparia, 

 Proetus, Remopleurides, Sphcerexochus, Staurocephalus, *Stygina, and 

 *Trinucleus, those with an asterisk not surviving this period. 



(3) The abundance of Cystideans, eight genera, and twenty-three 

 species, occurring in the Bala Group. 



(4) The abundance of Orthidse, and the appearance of the 

 following genera of Brachiopoda Atrypa, Leptcena, * Orthisina, 

 Rhynchonella, Siphonotreta, Strophomena, Trematis, and Triplesia. 



The occurrence of graptolites throughout the Ordovician System 

 in almost all areas where shales were deposited has afforded a 

 means of subdivision into stages or zones and of correlating the 



