Distribution of the Rhabdopkora. 25 



vertical range of the genus. The decrease in species as we 

 pass upwards through the Gala and Tarannon formations 

 seems to be very gradual. The recent researches of Dr. 

 Tullberg in the Cyrtograptus-schists of Scania * make it 

 very doubtful whether there was any marked falling-off in 

 species till far on in the Wenlock period. 



In the Lower Ludlow of Wales and Scandinavia indivi- 

 duals are still very numerous in certain localities, but the 

 species were few and of few types. Here our knowledge of 

 the genus suddenly fails us ; for although fragments of the 

 genus have been met with far within the limits of the Upper 

 Ludlow beds, no species has yet been identified. 



The several subgroups of Monograptus differ greatly in 

 their vertical distribution. 



Group i. [Nilssoni group) has by far the longest range. It 

 claims M. tenuis, Portlk., and M. attenuatus, Hopk., which are 

 the oldest Monograptidge yet discovered, as well as M. scani- 

 cus, Tullberg, and M. Nilssoni, Barr., which are the charac- 

 teristic forms of the Lower Ludlow formation. 



Group ii. {Hisingeri group) approaches the first group in the 

 great extent of its geological range ; but its oldest forms 

 descend no lower than the Middle Birkhill, while it is 

 doubtful if any true members of the group live on into the 

 Lower Ludlow. 



Group iii. (colonus group) is the last to come into being, as 

 it seems to be the last to die out. Its oldest known form 

 (M. galaensis, Lapw.) is found at the very base of the Gala 

 group. According to Mr. Linnarsson the fragmentary form 

 in the Gothland sandstones (Upper Ludlow?) is of this type. 



Its species are markedly characteristic of Upper Wenlock 

 and Lower Ludlow beds. Of its nine recognized forms eight 

 belong to these horizons. 



Group iv. (priodon group) contains many forms which, 

 according to the views of the Scandinavian paleontologists, 

 are of doubtful specific value ; and it is impossible, in the pre- 

 sent state of our knowledge, to separate its distinct species and 

 varieties with certainty. The group agrees closely in its 

 vertical range with the preceding. Its oldest form ap- 

 pears in the highest Birkhill beds, and its most recent in 

 the Upper Ludlow of Shropshire. It includes the only Mono- 

 graptus yet quoted from American rocks, which are remark- 

 ably poor in members of this family. 



Group v. (lobiferus group). This group is characteristic of 

 the Valentian or Llandovery-Tarannon rocks, no single 



* Tullberg, Geol. Foren. Forh. 1880, No. 59, Bd. v. No. 3. 



