638 J. HOPKINSON AND C. LAPWOETH ON THE GEAPTOLITES OF 



Two varieties of Didymograptus MurcJiisoni, viz. Didymograptus 

 virgulatus, Beck, and D. geminus, Hisinger, are abundant in the 

 possibly synchronous deposits of black Graptolitic shales containing 

 the Orthoceratite Limestone of Southern Norway. 



Several of the Rhabdophora met with in the Middle Llandeilo 

 beds of Abereiddy Bay are furnished by the corresponding calcareous 

 schists of Builth, Diplograptus foliaceus, D. tricornis, and Dicello- 

 graptus moffatensis being common to the two localities. A more 

 intimate connexion, however, is shown by the Lower Moffat beds of 

 South Scotland, in which all the forms as yet obtained from the 

 Middle Llandeilo beds of Abereiddy Bay are abundant and charac- 

 teristic. 



The Graptolites now known to occur in the Arenig and Llandeilo 

 Rocks of St. David's make up a total number of 45 species, belonging 

 to 14 distinct genera. 



Of these genera the suborder Cladophora claims four, viz. Ptilo- 

 graptus, Dendrograptus, Callogroptus, and Dictyograptus, all of which 

 appertain to the section Dendroidea. Some genera of this section 

 have been recognized in the Tremadoc rocks of North Wales and in the 

 equivalent deposit of the Potsdam Sandstone in Iowa. The whole of 

 them retain their distinctive characters, without material alteration, 

 from the Arenig rocks of Pembrokeshire to the Ludlow rocks of 

 Shropshire ; one at least occurs in beds of Devonian age in North 

 America ; and, as far as outward characters enable us to judge, they 

 appear to have their representatives in the seas of the present day. 



Although the Dendroidea in these beds are surprisingly numerous, 

 the genera constituting the section of the Thamnoidea are apparently 

 wanting. Examples swarm abundantly in South Scotland, and are 

 known also in the older shales of Point Levis : the St.-David's beds 

 however have not afforded us a single fragment. 



The suborder Ehabdophora, which includes all the more typical 

 Graptolites, makes its first appearance (as far as at present known) 

 in the Lower Arenigs of Ramsey Island, and finally dies out in the 

 Ludlow rocks of Shropshire. 



Of the genera belonging to this suborder found at St. David's, two 

 only, Trigonograptus and Glossograptus , appertain to the section 

 Retioloidea. Not a single specimen has been collected that can with 

 certainty be referred to the section Corynoidea — another striking 

 example of the apparently capricious distribution of some families, a 

 circumstance due in all probability to our present imperfect acquaint- 

 ance with these strata. 



The great section of the Graptoloidea claims the whole of the re- 

 maining eight genera. Exactly as in America, Cumberland, and Nor- 

 way, the family of the Dichograptidee is most fully represented. This 

 family, which is strictly confined to the Arenig and Llandeilo forma- 

 tions, is represented in the St.-David's rocks by three genera — its species, 

 with their characteristic long feathery branches, being first known in 

 Britain in the Lower Arenigs of Ramsey Island, and passing insensibly 

 away at the summit of the Llandeilo. The peculiar funiculate genera 

 are found in abundance in the Middle Arenigs of Whitesand Bay, 



