Distribution of the Rhabdophora. 187 



of characterizing a single subdivision only, it has a vertical 

 range at least as extensive as that of the entire formation 

 itself. The oldest form hitherto detected was collected by 

 Mr. Hopkinson in the lowest zones of St. David's. The 

 youngest species occur in profusion in the Phyllograptus-beds 

 (Upper Arenig) which overlie the Orthoceras Limestone of 

 Scania. 



Family vi. Diplograptidse. 



This family is especially remarkable for the great extent of 

 its vertical range. Its most ancient examples occur at the 

 very threshold of the Ordovician ; and its latest species do 

 not finally disappear until we reach the summit of the first 

 division of the Silurian proper. 



It is difficult to fix upon the exact horizon where this family 

 attains its greatest numerical expansion. In the Upper Llan- 

 deilo and in the Lower and Upper Bala it is equally prolific 

 in individuals ; several local horizons in each of these three 

 subdivisions could be instanced where examples are so abun- 

 dant that they almost hide the faces of the beds from sight. 

 In the earlier Arenig and Lower Llandeilo individuals are of 

 comparatively rare occurrence. In the later Lower and Middle 

 Llandovery, though they are less abundant as a rule than in 

 the Llandeilo-Bala strata, they nevertheless dispute the pre- 

 eminence with the Monograptidse. In the succeeding Taran- 

 non or Gala-Llandovery, specimens are generally few and far 

 between. In the Wenlock and Ludlow formations they are 

 wholly wanting. 



Diplograptus &c. — The two most important genera of the 

 family (Climacograpitus and Diplograptus) agree in their 

 vertical range with the entire family, appearing, culminating, 

 and disappearing together. It is impossible to decide which 

 is the more abundant. In the Bala rocks the numerical ex- 

 cess in species probably belongs to Diplograptus, in individuals 

 to Climacograptus. 



Of species, the form identified by myself with Diplograptus 

 foliaceus, Murch., has probably the most extended vertical and 

 horizontal distribution. It is found in abundance everywhere 

 in higher Llandeilo and Lower Bala strata. In the Lower 

 and Upper Bala its place is taken by D. truncatus, Lapw. * 



The subgroups Cephalograptus, Hopk., Petalograptus, 

 Suess, and Dimorphograptus, Lapw., are all exclusively Llan- 

 dovery in range. The beautiful forms Cephalograptus cometa, 

 Geinitz, and Petalograptus folium, His., mark the middle 

 zones of the Valentian throughout Western Europe and 

 Britain. 



