ADELOGRAPTUS TENELLUS (LINNARSSON, 1871) FROM THE LOWER ORDOVICIAN OF YIYANG AREA, HUNAN PROVINCE

Abundant, well-preserved graptolites were collected from the Lower Ordovician in the Nanba area of Yiyang, Hunan Province.A population analysis of Adelograptus tenellus (Linnarsson,1871) based on primary morphological characteristics such as the stipe length of each order,the number and arrangement of autothecae, and the thecal spacing realed two forms (Type Ⅰ and Type Ⅱ) in the Yiyang area. Comparing with areas such as North Europe, Great Britain and North American, an evolutionary lineage from Type Ⅰto Type Ⅱ of these two types is suggested.The results show that Adelograptus tenellus reported from Great Britain, New Zealand and Southern Anhui Province is similar to Type Ⅰ, whereas Adelograptus tenellus from Northern Europe shows a close similarity with Type Ⅱ.