24 Mr. C. Lapworth on the Geological 



Bala formation. The black muddy sea-bed that occupied the 

 region now covered in part by Britain and Scandinavia 

 seems to have been specially favourable for their develop- 

 ment ; and the increment in species and individuals is so 

 rapid, that by the time we reach the highest zones of the 

 Llandovery, the family has reached its specific maximum, 

 and possibly even begun its decline. The fact that the 

 succeeding Wenlock and Ludlow formations are more arena- 

 ceous in their character, and consequently allow of fewer 

 species being preserved to us, deprives this conclusion of some 

 of its weight. Nevertheless their most prolific zones, even 

 when lithologically identical with the teeming beds of the 

 Llandovery, are invariably so poor in the number and variety 

 of their species, that the highest Llandovery subformation may 

 safely be regarded as the metropolis of the family. 



1. Hastrites, Barrande. — This peculiar genus, so strikingly 

 individualized by its isolated hydrothecse, had but a very 

 limited existence. It is confined exclusively to the middle 

 zones of the Valentian formation, being absent from its 

 lowest beds, and disappearing before the highest strata are 

 reached. It apparently makes its earliest appearance in 

 the M. gregarius zone of the Upper Birkhill, where the 

 well-known species Rastrites peregrinus, Ban*., occurs in 

 great abundance, associated with its intimate ally, M. trian- 

 gulatusj Harkn. In the succeeding Upper Birkhill zones 

 other species come in, and the genus attains its maximum 

 development. In the overlying Gala Tarannon beds one 

 or two species occur rarely, and in the Lower division only. 

 In the Upper Gala the genus appears to have wholly dis- 

 appeared. 



2. Monograptus, Geinitz [restricted). — This is the richest 

 and most important of the three genera which compose 

 the family of the Monograptida? ; and its vertical range 

 is so extended that it is coextensive with that of the entire 

 family. The oldest fragments of Monograptus detected by 

 myself were found in the lowest (D. acuminatus, Nich.) 

 zone of the Lower Birkhill shale of Moffat. It is impossible 

 to refer these interesting fragments to any known species ; 

 but there can be no question that they are true Mono- 

 graptida?. In no other locality have examples properly 

 referable to the genus Monograptus been found so low in the 

 succession. 



The genus Monograptus possibly culminates along an 

 horizon which is identical with, or but a short distance above, 

 the boundary-line between the Llandovery and the Tarannon. 

 This is far below the line which marks the middle of the 



